Franz Ferdinand was the heir to the Austro-Hungarian Empire. He was assassinated by members of The Black Hand, a secret society of Serbian nationalists who wanted to unite all Slavic countries, which would first require independence from Austria-Hungary. The Black Hand had support from many higher ups in the Kingdom of Serbia, if not necessarily the government itself.
Austria-Hungary accused Serbia of supporting the assassination, and issued an ultimatum with very harsh terms to Serbia to “investigate” but would actually put them into the A-H sphere of influence.
MEANWHILE, Russia had recently suffered a humiliating defeat in the Russo-Japanese War, and was looking to restore its prestige and influence. The Tsar decided he would be “Protector of the Slavs” and thus back Serbia against Austrian aggression. This gave Serbia confidence to defy A-H.
MEANWHILE Germany had an alliance with A-H, and had told them basically “we trust you, do what you want and you have our support.” This gave A-H the confidence to threaten war even if it would anger Russia.
MEANWHILE the unification and rapid rise of Germany in the previous decades had the other European powers very nervous, and France and Russia made an alliance to protect themselves in case one of them had to fight Germany. This was also supported by Britain, though not officially. France in particular was arguably looking for a reason to fight Germany.
SO:
A-H threatens Serbia, Serbia refuses. A-H declares war on Serbia. Russia declares war on A-H. A-H calls for help from Germany. Germany declares war on Russia, which also means declaring war on France because they are allies.
Germany decides to attack France first because they think Russia will be slow to mobilize. They move their army through Belgium to get around the border defenses in France, expecting Belgium to simply let them rather than fight. Belgium does NOT let them! Belgium calls for help from Britain, citing an old treaty that supposedly guarantees their independence. Britain considers Germany their greatest rival, and so decides to come to Belgium’s aid.
Italy and the Ottoman Empire both see the chaos in the Balkans and each decide to take advantage to gain more territory, joining the war on opposite sides. (Italy had been allied to A-H before but invaded them anyway, since that was the territory they wanted.) Japan did the same thing, except in the Pacific, since Germany had no means to protect its colonies while Britain had naval supremacy.
MEANWHILE, the United States was busy loaning money to fund the war, primarily to Britain. Germany was pissing them off because their naval strategy relied on submarines to attack merchant shipping, which ended up killing Americans. Germany feared the Americans would join the war against them and tried to recruit MEXICO to invade America to keep them busy. When this plot was discovered, America finally had enough of neutrality and declared war on Germany too.
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