Treaty of Smarba

The Treaty of Smarba was signed in 2266 between the Klingon Empire and the Romulan Star Empire. (Star Fleet Battles)

Despite incidents such as the Battle of Klach D'Kel Brakt, it was considered to be voided only by 2292, at which time the Romulans were considered blood enemies by the Klingons. 

Terms

 * There shall be unrestricted technology exchanges between all the parties
 * The Klingons and the Romulans shall jointly attack the ­Federation

Consequences
The Klingon Defense Force acquired cloaking devices, while Romulans acquired new-generation warp cores, as well as starships. 

As part of the Treaty, the Romulan Guard received Klingon-built hulls but at an exorbitant cost. One of the Treaty's provisions was never fulfilled, that is, the delivery of a battleship to the Romulan Guard. (Star Fleet Battles)

Production notes
This treaty was simply there to explain why the Klingons had cloaking devices by 2285.