Assassination of Archduke Franz Ferdinand (1914).
World War 1 Begins.
It is said to be the spark that started the global war.
Germany though that they can just march through Belgium for a surprise attack on the north of France, but they were surprised to find liege was circled by twelve heavily armed forts that held them off for 12 days.
Unrestricted Submarine Warfare.
Russia was in the middle of their own Revolutionary War so it became hard for them to also fight another war at the same time. So they signed a treaty with Germany
Treaty of Versailles.
Soldiers in the trenches endured conditions ranging from barely tolerable to utterly horrific.
Unrestricted Submarine Warfare was a result of desperation and the belief that the ferocity of such a tactic might just keep America out of the war if the results were spectacular and shocking enough.
Treaty of Versailles was the most important of peace treaties that brought world war 1 to an end.