In a lot of ways, 1917 was the decisive year; after the utterly futile battles of the Somme, Verdun, and the Brusilov offensive, the Allies started to (and only really then started to) get their shit together.
1917 saw the entry of the US into the war, the first use of tracked, armored fighting vehicles (tanks), the two Russian revolutions.
Lots of stuff to come.

__________________
On the kinkometer, my kink measures as a sine wave.