Kakashi's Gaiden - The Battlefield of a Boy's Life!

"More than ten years ago, the rule of the Five Great Shinobi Nations was destabilized, throwing the world into a conflict later to be known as the Third Great Ninja War. The future Fourth Hokage Minato Namikaze assigns young Kakashi, Obito of the Uchiha clan, and Rin, a medical ninja, on a mission to Kannabi Bridge..."Source: crunchyroll