While the build-up is a slow burn involving political maneuvering and deception, the war has a definitive starting point.
Naruto eventually learns of the war and escapes Turtle Island to join the fight, leading to some of the most iconic battles of the series.
For anime viewers, the Fourth Great Shinobi War officially begins in Episode 261 , titled "For My Friend." *
"Tobi" (Obito Uchiha) officially declares the Fourth Shinobi World War against the Five Kage during the Five Kage Summit after they refuse to hand over Naruto and Killer B.