I heard Archiduke Ferdinand tripped and caused it. Your thoughts?
A/H wanted to humiliate Serbia who was competing for the spoils of the collapse of the European Ottoman Empire. So they happily gave Serbia a list of demands that couldn't be met.
Germany backed A/H with secret discussions.
Russia backed the Serbains and mobolised in a show of strength but found to it's dismay that it had no plans for partial mobilisation against Austria only and had to mobilise against Germany as well.
Germany, faced with the Russian mobilisation, needed to mobilise as well or risk being defenceless. The problem was the German plan for mobilisation involved massing most of the army on the French/Belgium border to knock the French out before the slower Russian armies could complete their mobilisation. This plan (the Schlieffen plan) involved a drive through Belgium to take the French on the flank.
Britain, who had been in a naval arms race with Germany since the Kiel channel was widened to allow Dreadnoughts, veiwed the occupation of Belgium with nothing short of horror and joined in. They also kept two battleships that Turkey had ordered and payed for "for the duration".
Germany then offered two ships that were in the Med as compensation to the Turks and, with German sailors decked in fez's promptly shelled Sevastopol bringing Turkey into the war.
Long story short - what should have been a minor war between A/H and Serbia ended up destroying 4 empires and the 19th century status quo (along with millions of lives) because the Great Powers of the time couldn't help but stick their nose into everybody elses buisness.