Question

How did forging alliances in became one of the reasons WW1 started?

Answer 1

Because they all call each other to war.

Explanation:

Let me give you a brief time-lapse of the events. Taking into account that Britain, France, and Russia are allied, and on the other side there was Germany, Austria-Hungary, and Italy*

-Franz Ferdinand is murdered by a Serb (Gavrilo Princip) on June 28th, 1914.

-Austria-Hungary declares war on Serbia on July the 28th, with German backing.

-Russia was guaranteeing Serbia, and thus begin mobilization on the 31st of July

-On August the 1st, Germany declares war on Russia, which prompts France & Belgium to mobilize as well.

-On the 3rd Germany declares war on France and Belgium, Britain was guaranteeing Belgium, and sends Germany and ultimatum to not invade Belgium, but Germany rejects it.

-August 4th, Britain declares war on Germany, which forces all of the British dominions to join in (The Raj, Canada, Australia, etc.)

-August 6th, Austria-Hungary declares war on Russia.

-August 12th, Britain and France declare war on Austria-Hungary, and Serbia is invaded (by AH).

By this list of events, it is clear how the alliances were like a sort of mechanism, where this triggers that, which triggers this, and so forth.

*Italy had joined the Triple Alliance (Central Powers) on 1882, but remained neutral in 1914, and then signed the London Pact in 1915, which then led them to declare war against Austria-Hungary, but not Germany.

Source: http://www.historyplace.com/worldhistory/firstworldwar/index-1914.html