Question

What strategy did Otto von Bismarck use to make Prussia the leader of a united Germany?

Answer 1

Realpolitik

Explanation:

First and foremost, it is important to analyze how the German Kaiserreich functioned when it was Prussia. So, there was Wilhelm I, the king, and then there was Otto von Bismarck, a highly trusted confidant of the king. Important to discern that whilst Wilhelm I was king, Otto wielded all the foreign diplomacy, this was his boon in order to promote his own view for a greater Germany.

Therefore, he devised this clever system of international affairs, (realpolitik), rather than basing the international status of another nation via morals, he based it off of "realistic" factors, such as economy, army size, etc. He managed to therefore ally Russia and Austria Hungary, securing his eastern borders completely, allowing for the expansion of Prussia into the mainland. Now the way he did this, was extremely clever, in the sense that he manipulated several elements of diplomatic actions, in order to incite war.

This, of course happened in 1870, where Bismarck remitted a message sent by the French, and responded in such a way which infuriated the French, and made them declare war on Prussia. Bismarck, knowing all too well how much more advanced Prussia was than France (in the beginning France had more advanced rifles, but other Prussian inventions, turned the tide; Prussia also had a militaristic tradition by the way). So when France declared war, Bismarck, after defeating them, managed to unite (most) of Germany under one banner, under pretexts and such.
Aftermath:

After Wilhelm II took power, he kicked out Bismarck, and went fully berserk on other German nations (proclaiming that Germany had a right to them), which scared Russia off the alliance, and then joined the Entente (Great Britain and France).