Star Trek

So, as seen throughout Star Trek: The Original Series, the Federation and the Klingons are in a sort of "Cold War". War had been declared, and there were some battles between the two.

However, inThe Next Generation, a Klingon serves as chief of security aboard the USS Enterprise.

What brought about that peace? Was there a treaty or something? I've looked all over the internet, but I couldn't find an answer, and I was hoping someone could help me.

It's explained in Star Trek VI: The Undiscovered Country - if you haven't seen that film yet, I'd suggest watching it before reading any other replies in this thread as they will probably contain spoilers for it.

A Klingon moon exploded, The Federation said, hey, the Klingons said hey back and they signed a peace treaty on Khitomer.

Worf is an exception though. No other full Klingons would ever join Starfleet, it's a child's uniform!