Make the internet a better place to learn

Well, the main contendent were:
The South Korean and US army leading the UN forces (comprising many countries: United Kingdom, Australia and France among others);
The North Korean People's army;
The Chinese People's army.

The war developed through three main stages:

I) North Korean invasion of the south (25 June-September 1950); occupation of the capital Seoul and maximum penetration into the south (Pusan and Pusan perimeter);

Build up of UN forces (mainly US, armored and air force) and consequent strengthening of the opposition to the invasion with counter-attacks and heavy bombing of supply network of the North Korean force leading to a virtual halting of the momentum of the invasion.

II) US Counter-attack (September-October 1950); amphibious landing at Inchon, near the capital Seoul and sortie of mechanized troops from the besieged Pusan perimeter. At this point the North Korean army poorly led for defensive action and lacking naval and air support disintegrates.

US forces rapidly defeat the North Korean units recapturing the capital and even penetrating into North Korea (crossing the 38th parallel). In October the North Korean capital, Pyongyang, falls and US troops charge towards the Chinese border in pursuit of North Korean leaders.

III) Chinese intervention (19 October 1950); crossing of the Yalu river by huge numbers of "volunteers" from the Chinese People's army in "patriotic" support of the North Korean Army. Bitter fighting against US formations using the "en-masse/cannon fodder" technique by the Chinese troops causing widespread heavy casualties and consequent retreat of US formations.

After various battles and even the consideration of deploying nuclear weapons the war came back to where it started, i.e., around the 38th parallel. For many months several battles are fought mainly between Chinese and US forces with little or no gain of territory (kind of re-edition of WWI trench warfare).
This situation led to a stalemate and consequent armistice (July 1953).

Trenches are a form of protection/fortification for the soldiers allowing them to get shelter from shells, bullets, splinters and, basically, enemy fire.
On the other hand if you are the one attacking a trench, as in WWI, probably you'll have a bad day!
The same characteristic that makes a trench a very useful protective structure makes it a very dangerous and potentially deadly enemy emplacement.

First of all a trench has firing positions where soldiers use the protection offered by the trench to fire to incoming attackers (normally using a machinegun!);

Then the trench is built to withstand shelling so that soldiers are protected during the enemy opening artillery attack and come out when the infantry attack starts;

Finally the trench is cleverly built to produce as much damage as possible to the attacker: it comprises barbed wire "fences" to force the attacker to stop and get more vulnerable; minefields with paths through them to "channel" the attackers into killing zones (paths leading to machineguns); their geometry tends to separate the waves of attackers in chunks and to disorientate the soldiers to be easily eliminated by the defenders.

There is also a specific set of problems for the soldiers actually using the trenches for defense and mainly related to sanitation hazards: the trench, in particular during WWI, was not a very clean or healthy environment.
You had:

1) Dampness and cold: causing diseases and damaging the skin/limbs (trench-foot);

2) Parasites and vermin: the trenches are perfect environments for rats, lice, fleas and all the other infesting insects present in a dirty environment where (not washed) people have to live in close contact;

3) Bodily fluids. Evacuation of human feces is problematic as it is for the bodies of dead soldiers and shelling, for example, can disperse them or even rotting flesh on soldiers causing diseases.

These are only some of the dangerous characteristics of trenches that an attacking or defending soldier had to face.

In total Germans and French lost during the battle about #500,000# men (or more) and while for the French was a "glorious" victory (they were able to defend and retain a place of historical importance but strategically irrelevant) for the Germans was the first serious defeat of their mighty and proud army.

Also, for both the contenders was a battle that decimated their armies without giving a decisive victory for the outcome of the war.

The initial idea of the German commander in chief Falkenhayn was to nail down the French in a position they were forced to defend at any cost (for reasons of historical and of national pride, the fortress of Verdun) and in doing so get bled to death loosing as many soldiers as possible (battle of attrition), annihilate their army and possibly be forced to sue for peace.

The French, under Petain, were forced to defend every inch of ground suffering terrible casualties. The horror of the battle was magnified by the extensive use of poisonous gasses, flamethrowers and, obviously, extensive and devastating use of artillery that basically created a "moon" landscape on Earth!

At the end nobody really "won" as we are used to think of a normal battle: the Germans were repelled and went back to their starting trenches but the "victorious" French paid a terrible price, that greatly damaged their army, to defend a place that was, basically, strategically useless.

It is quite difficult to exactly define the real influence of Rasputin on the Czar family or Russia and to separate reality from the legend created around this weird figure.

For sure it became a kind of confident/healer of the Czar family possessing magical or mystical powers that allegedly allowed him to alleviate the sufferings of the hemophilic son of the Czar, Alexei.
His influence was particularly strong on the Czar's wife, Alexandra, convinced that Rasputin, in several occasions, saved (magically) her son's life.

Due to the support of the Czarina and indecisions and incapacity of the Czar, Rasputin became more and more involved into politics, trying to influence governmental decisions and even military ones.

It is difficult to define the real damages caused by Rasputin in a period where in Russia everybody was against everybody in trying to damage as much as possible each other.

The reality was that the Army was poorly led and equipped (the generals were responsible for the few victories but in case of defeat it was always the Czar fault!), the politicians were quarrelling continuously among them with no regard of the nation's sake and the nobles lived an anachronistic medieval life of excesses and debauchery.

My sensation is that, yes, Rasputin was a weirdo with a strong influence (sometimes bad) on the decisions of a ruling Czar (that actually ruled very little) but certainly he was used as a scapegoat for the disastrous global situation of Russia at the time!

England: English settlers explored and colonized the East Coast of modern-day America and Canada, with some minor exploration of the Caribbean region and even Pacific islands like Hawaii (although this was in the 1700s).

Portugal: This tiny nation kicked off the Age of Exploration with their journeys around Africa. Portugal got a little shafted in the New World though; the only major colony they picked up was Brazil.

Spain: Spain basically explored and settled everywhere south of Mexico. In South America, they conquered the Inca. In Mexico, they conquered the Aztec and Maya. They were the first to see the Pacific Ocean going from east to west, across the Atlantic. They got the biggest chunk of the pie - pretty much all of Mexico and Western South America. Their epic silver mine at Potosi nearly crashed the value of silver, because there was simply so much of it!

The Netherlands: One of the minor players. They're mostly known for their colonization of the East Indies, i.e. Indonesia, but also picked up a few colonies in the Caribbean. They also did some exploring in the New England region and are famous for purchasing Manhattan Island from Native Americans for $24 (though it may have been a little more than that).

France: French explorers were pioneers, exploring the Ohio River Valley and trading furs with the natives. Their colonies at one time included some Caribbean Islands, like Haiti, and the Mississippi River Valley (the latter of which was sold to America in 1803 for $15 million).

I would choose "unrestricted submarine warfare".

Chemical warfare was first introduced on the battlefield of WWI by the Germans during the battle of Ypres.
The problem is that this was not very new, in fact, it was even contemplated (and forbidden) by the Hague Convention.

Basically, poisonous gasses were primarily used on the battlefield against soldiers, and both sides had chemical weapons. In theory the civilians were not supposed to be affected by this (this is only the theory, considering that the war was certainly fought near civilian areas anyway!).

Submarines on the other hand were considered "barbaric" because they would strike against military or civilian ships (Lusitania) regardless of the fact they had women or children on board ( unrestricted submarine warfare ).

The unfairness was considered because you could not see a submarine and so fight it (that, after all, was the idea of using submarines in the first place!) and so not very...sporty!

Submarines and submarine warfare was considered badly by "traditionalist". You can see this considering that up to the present day British submarines carry a "pirate" flag or "Jolly Roger" to remember that, at the start of their deployment in WWI, the high command considered them as pirates so that Admiral Sir Arthur Wilson, the First Sea Lord of the British Royal Navy, stated that submarines were:

" underhanded, unfair, and damned un-English", and that he would convince the British Admiralty to have the crews of enemy submarines captured during wartime be hanged as pirates ".

In the year 1900, an unprecedented alliance occurred between the eight major military powers of the world: Austria-Hungary, France, Germany, Great Britain, Italy, Japan, Russia and the US.
Never before and never since has there been such an alliance fighting against a common enemy, a secret society whose goal was to exterminate all the "foreign devils" in China.

This society and its fighters came to be known as The Boxers (probably due to the fact that the westerners often saw them doing physical exercises and Tai-Chi, considered close to boxing).

In reality the Boxers were mainly peasants of the poorest regions of china that thought (or were led to believe) that the cause of their poverty were the westerners (actually even without westerners probably they would have been in the same conditions anyway under the domination of local warlords).

The Boxers had few and very old traditional Chinese weapons or were armed with agricultural tools (forks, spades, clubs...). They also lacked military training and discipline so that their attacks resembled more hooligans’ brawls than military planned operations.
To alleviate the lack of armament they were even induced to practice magic rituals that would protected them from bullets...!

The leadership of the Boxers was also a little bit confused. Apparently they were led by Prince Tuan guiding the Boxers towards the just emancipation from the occupation of the westerners. In reality, I think, they were puppets in the hands of the Qing Empress Dowager Tzu’u Hzi. She was acting in a very devious way: if the Boxers had won she could say that from the start she had supported them to the point of involving the regular Imperial Army to help them; but if the rebellion had failed, then she could (and did) present herself as an innocent bystander involved against her will in the fighting only to defend Chinese territory from the landings of foreign troops (attempting to relieve the besieged western legation in Beijing).

Soon the Boxers became too focussed upon eliminating the foreign ambassadors in the legation quarters of Beijing (using mainly suicidal frontal attacks) to pay attention to the fact that strong military contingents of the western nations were landing nearby and converging on the capital to crush them. They left the job to stop these relief parties to the regular Imperial Army (lead by General Jung Lu) whose generals were not exactly thrilled to work for the Boxers!

As more and more western troops started to land and move to relieve the legations in Beijing the situation of the Boxers became more and more dramatic. Their fruitless frontal attacks against the combined armies in Beijing (small but well organized and highly trained) were rapidly bleeding them white. The regular army was not really helping them.
As the foreign relief armies fought they way to the capital the Boxers' forces started to melt away into the countryside.

At the end of the day the Boxer Rebellion did exactly the opposite of what was intended to do:
It united its enemies in a strong and formidable coalition (the western powers) and divided the Chinese forces (the boxers' leaders against the Imperial Army generals against the Empress and all against each others) producing a fragmented and higly vulnerable force.

[References:L. E. Bodin, C. Warner The Boxer Rebellion. Osprey Men-at-Arms Series, 1979]

In 1933 the National Socialist (Nazi) Party came to power in Germany when its leader, Adolf Hitler, became Chancellor.

One of the first steps, to ensure that no opponents could interfere, was the introduction of a "one party" structure excluding from the Parliament (or reducing) the presence of other parties representatives such as communists, socialists, social-democrats, etc. sometimes even physically attacking them.

This was possible and done through a clever activity of propaganda accusing the "decadent parties" to endanger Germany with their actions and so not being worth to represent the people in parliament:

the communists because connected to Soviet Union and so accused to be serfs of Stalin and trying, with their actions and ideal of revolution, to handle Germany to him (this was particularly appealing to the rich and industrialists);

the socialists and social-democrats because guilty of always quarreling in parliament without reaching any conclusion or solving problems and leaving Germany in a state of defenseless and prostration as after World War I (this was particularly appealing to the masses left enraged after the capitulation and induced to think that they were betrayed by the weak parties in Germany).

But Hitler didn't have enemies only in other parties.

The Nazi paramilitary faction of the SA (SturmAbteilung - Action Squads) lead by Ernst Rohm represented a big danger for the political ambitions and position of Hitler. In 1934 Hitler unleashed the SS (envious of the power of Rohm and lead by his antagonist H. Himler) in a purge of the SA. During the Night of the Long Knives Rohm was arrested and murdered and countless other SA members were eliminated throughout Germany. After this night the SA became a kind of a folkloristic parade formation only used during Nazi celebrations and Party rallies to remember the good old days of Nazi history.

In general, opponents to the Nazi were excluded from prominent position and their names, occupations, skills and origin recorded in lists that later were going to be used to round them up to be sent to concentration camps and….disappear!

These actions were implemented even at a person to person and local basis. I remember an anecdote of a German friend of my father (that witnessed that period) that told us this story:

Throughout Germany functionaries of the government were “censing” the owners of radios (at that time comparable, as influence, as today’s TV) asking also if they wanted to change their “old and battered” sets with a new one completely free supplied by the state (obviously this new and free set couldn’t really get ALL the radio stations…its frequency selectors were set!). The majority considering the kind of “mafia” stile request and imagining the consequence of a refusal obviously accepted the new set. When the functionaries met someone that didn’t want to change his radio simply and friendly told him: “Sir, could you give us your name, address and occupation….only for the record, you know…”

• Best book: John English, "A Perspective on Infantry", an excellent survey on the history of tactics in the 20th Century.

The German Army thought hard about what the British and French did to them in 1918 (when even the toughest trench-lines were quickly penetrated) and drew the appropriate conclusions about tactics, training and organization. Success lay with far more than just new weapons like dive bombers and tanks.

Old ideas (going back to Napoleon) were combined with new training so that everyone from privates to Generals made decisions quickly and moved decisively. The idea was to keep the enemy off balance, and unable to react except to circumstances that had already changed. Instead of breaking his will by attrition and sheer destruction, the enemy's morale was the target -- to make him dazed and confused until he was trapped or cornered.

Radios made it easier to coordinate, and -- unlike in WW1 -- trucks and armour could move much faster than ever. What was more important were "mission tactics: (Auftragstaktik)... soldiers knew what their objectives were, but were free to work to achieve them using the best means at their disposal and the resources they were given.

Speed and surprise were vital. Soldiers were also supposed to improvise and adapt to any circumstances. Making a poor decision immediately was better than making an excellent one later. Tough problems were to be screened off and handled later, weak points were to be exploited immediately.

Besides the Germans in Poland, Norway and France, this is also how the British destroyed an Italian Army in Libya in two months in 1940-41. The Soviets had similar plans, but it took them two years to have officers and men capable of fighting this way.

The Prime Ministers of Japan (all of them of the armed forces) were really leading Japan and the war effort.
As Prime Ministers, we had: General Hideki Toyo (from October 17, 1941, to July 22, 1944) followed by General Kuniaki Koiso (from July 22, 1944 to April 7, 1945) and finally Admiral Kantaro Suzuki (from 7 April-17 August 1945).

Toyo was the leader during the attack against Pearl Harbor (the episode that started the war against the US). He was considered the Japanese version of Hitler/Mussolini by the Allies.

When the war started to become really difficult for Japan Toyo simply resigned and Koiso was chosen as a kind of figurehead to let the army going on in its suicidal course of total war.
When the war went even more badly Koiso resigned and was followed by an Admiral of the Navy (up to then considered “dishonored” after all the naval defeats suffered starting from Midway).

Suzuki, basically, managed only to negotiate for peace (risking his life in the process) against the will of the fanatics of the army that wanted to continue the war to win better conditions of peace.

[Hideki Toyo]

As my personal input I think that the Emperor wasnt really extraneous to the crucial decisions that led to the war and its disastrous conclusion. After the war, to avoid problems with the population, the Emperor was presented as an innocent bystander that couldnt do anything to avoid or stop the war. In reality after the two nuclear bombings and the declaration of war from the USSR, he IMPOSED the armistice to the army showing that he wasn`t really a simple iconic figure without power!