Bousou Sengokushi

Modern-day bikers make and break alliances with neighboring gangs, fight over the right to use a local petrol station, and are eventually united under a powerful leader. It's a replay of the events of Japan's 16th-century civil war, but with bikes instead of horses and knives instead of swords. Based on the manga by Jiro Ueno in Weekly Playboy.

(Source: The Anime Encyclopedia)