The Human-Klingon War was a conflict between the Human Race and the Klingons. The war started when the Klingons got irritated because Spock knowns everything, and declared war on the Federation. The Klingons were utterly destroyed by Spock's mind powers, and were thusly relegated to a mere three ships in their once vast navy. As the Human fleet surrounded their home planet of Kronos, they quickly drew up a fair peace treaty which the Humans graciously accepted.
Since then, the Humans and the Klingons have been close allies, because, like the Elites, the Klingons have a great amount of respect for powerful races who are worthy in battle.