Prologue: Fourth Great Shinobi War
The Fourth Great Shinobi War is raging on without Naruto even being aware of the fact that his world is at war to protect him and Killer Bee! He's unaware because of his training to control the Nine Tailed Demon Fox's massive amount of chakra on an uninhabited island.
He only realized that the war was going on when he sensed great amounts of chakra clash against other equally strong amounts of chakra. What really got him confused was that he sensed the chakra of two of the members of the Akatsuki that he personally killed with his own two hands. He knew that they were dead.
When he got to the boundary of the island, he leaped. He crashed into a barrier, which he soon discovered was meant to keep him there., which could have only meant that the whole war was to capture the tailed beasts that were sealed in him and Killer Bee. Their villages didn't want to risk using their power because that would make it easier for their opponents to capture them.
This barrier can't hold me here long! I will smash it and join the war to prevent my friends from dying. That's a promise! Naruto thought to himself.