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.
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
Soldiers in the trenches endured conditions ranging from barely tolerable to utterly horrific.