All these soldiers that fought for our country knew that there was going to be a huge chances they weren't going to ever see their families ever again.
Nationalism
During WW1 both sides of the war had alliances. Germany had Austria-Hungary, Bulgaria, and the Ottoman empire. The United States had France, Japan, Britain, Russia, and Italy.
The assassination of Archduke Ferdinand
They were all fighting for more land and more natural resources. also more land for industries, and things like that.
The countries were being super greedy and not negotiating about it and
the assassination of Archduke Ferdinand and his wife Sophie was the spark of this great war. they were shot driving by Gavrilo Princip on the Latin bridge.