Basically, the assassination was carried out with weapons supplied by a Serbian intelligence organisation (who may have been slightly exceeding their authority). Austria-Hungary was understandably pissed that they had done this, and ultimately declared war.
Russia had a treaty with Serbia, and declared war on Austria-Hungary in turn, but that was okay because Austria-Hungary had a treaty with Germany, and thus Germany declared war on Russia.
At this point Germany sends a message to France saying “please remain neutral so we can go fight Russia without worrying about you”, and France said “no, we’re mobilising”.
Germany figured France was just waiting for Germany to move their troops East so France could invade Germany from the west, and decided to pre-empt this by invading France, the best way to do so was by going through Belgium.
Britain had a treaty with both France and Belgium, but was tempted to stay out of it and let the French get clobbered, but not Belgium. To the surprise of Germany, Britain declared war.
Then the Ottoman Empire got German help to invade British territory in the middle east to cut off British oil, Italy got bribed to fight on the side of Britain, France and Russia.
Germany had been deploying submarines to disrupt British trade, and after a few years this pissed off America so much they decided to deal with Germany.
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