Archduke Franz Ferdinand, heir to the Austria-Hungary throne and his wife were assassinated by a Slav. WWI officially started when Austria-Hungary declared war on Serbia.
Germany agreed to help Austria-Hungary in WWI because they had an alliance, along with Italy (Though Italy did switch sides during the war.) Together, they formed the Triple Alliance while France, Russia, and Britain formed the Triple Entente.
WWI had severely more casualties than any war fought before because it introduced new technology, including trench and aerial warfare.
The USA finally joined WWI when news of the Zimmerman Telegram was discovered. They supplied the Triple Entente with tons of troops, supplies, and cash which ultimately defeated the other side. Russia also left the war because of the Russian Revolution.
The Treaty of Versailles was signed which marked the end of the war. It blamed Germany for WWI. Of course, they were unhappy about this because it required them to give up their resources and even made them suffer from an economic collapse. The United States' Woodrow Wilson tried to negotiate but nobody tooked them seriously when they weren't part of the situation in the first place.
Even after the war was over, the damage was still present. The countries involved suffered from loss of population, economics, and property. Soldiers who belonged to the Lost Generation also faced shell-shock, a form of PTSD from war. Culture changed, which included new ideas of art, music, and literature.