1: The failure of the royal family in Russia, that helped lead to the Bolsheviks taking over Russia, and changing Russia into a communist country, renamed USSR.
2: The sinking of the Lusitania and the Zimmermann note brought the US and its fresh supplies of troops, ammunition, food, etc, into the war, and with Germany losing allies due to them surrendering, and they losing too much troops and couldnt keep up with the production of the Allies, they started to retreat, and by 1918, they surrendered