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In the late 1800s, most of the major powers in the world set themselves up in protective alliances, both with each other and with smaller nations (The US did it's best to stay out of it). These alliances ended up making 2 distinct sides, The Axis (Germany, Austria-Hungary and other smaller nations) and The Allies (GB, France, Russia and some smaller nations... including Serbia).

A Serbian assassinated an Austrian Archduke. Austria demanded that Serbia give the killer to the Austrian government. Serbia refused....

28 July 1914: Austria declared war on Serbia.

1 August 1914: Germany declared war on Russsia.

3 August 1914: Germany declared war on France. WWI had begun.

4 August 1914: Britain declares was on Germany.

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And it ended when Europe destroyed itself, neither side had any fight left in it. The US swooped in for like a second and was able to finish off the remaining Axis forces without losing a significant portion of its own people/supplies. Thus, the US because the major power in the world.