WWI started in the "powder keg," or the Balkan peninsula(sp?). Austria-Hungary had a region called Bosnia. Bosnia, Russia, and Serbia were mostly slavic. Serbia wanted Bosnia, and Bosnia wanted Serbia. Franz Ferdinand, the Austro-Hungarian heir was assassinated by a member of the Black Hand, which is a Serbian terrorist group. Austria-Hungary (A-H) couldn't do anything on their own because Serbia's "big brother" was Russia, so Germany sent A-H a "blank check" saying that whatever A-H does, they'll back them up. A-H then sent an ultimatum to Serbia and Serbia fulfilled all of the requirements except to allow A-H troops into Serbia to hunt down the assassins. A-H and Serbia started fighting. Russia started mobilizing its troops because they were really slow, and Germany took that as a hostile sign and declared war on Russia, but Russia was allied with France so Germany declared war on France because they knew they'd be fighting them anyway. Germany was facing a 2-front war, a country's worst nightmare, but Germany had defeated France in about 8 weeks in the Franco-Prussian war a few years earlier and thought they could do it again. So they sent troops into France and left just a few at the border b/t Germany and Russia. But France wasn't easy this time. They had begun trench warfare where pretty much nobody goes anywhere. So Germany tried to go around the trenches and come up behind the French army, but to do it they had to go through Belgium, which was neutral and England's "little brother" so-to-speak. So England entered WWI because of the German march through Belgium.
The quick and simple answer: the alliance system...
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I have a wonderful Modern European History teacher. :-)