If you're interested in re-watching or reading about this exciting arc, here are the key episodes:
– Tobi (Obito) officially declares war on the Five Kage after they refuse to hand over the Eight and Nine-Tails. which episode 4th ninja war start
The Fourth Shinobi World War is the defining climax of the Naruto Shippuden series, bringing together thousands of shinobi from all Five Great Nations to face the threat of Madara Uchiha and the Akatsuki. If you are looking to jump straight into the action, The Official Start: Episode 262 If you're interested in re-watching or reading about
The final battle against the progenitor of chakra. which episode 4th ninja war start
If you want the full context of how the war was declared and organized, you might want to start a bit earlier: