It was a fragile alliance. Gowron was rightly mad when the Federation refused to help out in the Klingon Civil War. Even their indirect aid was something of an insult. DS9 later showed how fragile the alliance was. When the Klingons felt like turning to expansionism again, and invaded Cardassia, the Federation condemned it and warned Cardassia. That's when Gowron broke the alliance and the Klingons and Federation started to engage in a quasi-war with them. It was only when Cardassia joined the Dominion that the alliance was renewed. A blatant act of convenience.