Reply #11: The system of alliances formed after the Franco-Prussian war...

[Spider Jerusalem]

is mostly to blame. Austria used the assassination of Franz Ferdinand as an excuse to declare war on Serbia; Russia saw this as a violation of its sphere of influence, and there was a general mobilisation of Russian troops. The Germans became rather nervous at this, because they'd WANTED war anyway, and expected to have to fight both Russia and France, but their plan for fighting on two fronts was predicated on the Russians being disorganised and slow to mobilise. So following the tsar's rejection of a German ultimatum to stand down, Germany declared war on Russia, and then the next day on France (the French had mutual defence agreements with Russia, primarily to act as a check on German aggression following the Franco-Prussian War), and when war came, the Germans were determined to invade France first, as the French army was the numerically inferior of their two opponents. The most direct route for the invasion lay through Belgium, which brought Britain into the war because of their obligations, under the 1839 Treaty of London, to protect Belgian neutrality. Really, the whole thing was just a monumental cock-up.

I'd recommend that you read The Guns of August, by Barbara Tuchman; it's one of the best books on the beginning of the First World War.