^ Well we know the Klingons and Cardassians fought an 18 year conflict over some nebula. That may very well overlap and be related to the Federation's war against the Cardassians. Although Garak described the Klingon war as "ages ago" that could mean anything. Especially since he also described the 18 year long conflict as a "minor skirmish"
Also to the Klingons' credit, they did send warships to engage the Borg when Federation was invaded. They also sent three warships to cover the Enterprise on a preemptive strike into the Romulan Neutral Zone, when Starfleet's own reinforcements were still trailing behind.
And of course when the Dominion first deployed their fleet to the Alpha Quadrant, the terms of the original treaty was brought back to life so that the Federation and Klingon fleets could form a combined task force
I'd say the Klingons did their share. It was the Federation that refused Klingons' request for assistance. First time was as we discussed, in the civil war with Duras which Gowron just shrugged off since he knew it was an internal conflict and the treaty wouldn't apply. But he doesn't lose anything by asking anyway. And second, when the Klingons invaded Cardassia, they did thinking they were stopping a civilian uprising that was orchestrated by the Founders (which if it had been true does kinda represent an external threat). This time Gowron was like WTF, you guys are not helping again? you guys are pussies. Take your diplomats and GTFO. Treaty is over