I like to theorize that the reason
the Feds are losing the war in the Alt Timeline is because, by their very nature, the Klingons bounced back into "war mode" much quicker than the Federation did. So Starfleet probably were on the back foot from the very first moment official declarations of war were made. Both fleets had to demilaterize following the events of STVI, but the Klingons are a warrior people and their ships are much smaller and more "rough and ready" than Federation ships, ergo also easier to mass produce. With this in mind no doubt the Klingons regained a war fleet that was on pre-peacetime levels much faster than Starfleet could rebuild their own armada.
As a kind of thumbnail sketch, here's how I imagine the sequence of events:
Following the events of STVI, both sides demilitarize. Parts of both fleets are stood down, but an uneasy
peace stands for a while on both sides, because it's impossible to forget past conflicts so easily. Perhaps the traitors from STVI agitate things a bit as well from behind bars.
After several decades of this kind of uneasy tension, comes Narendra III. This is the turning point which decides whether the future turns into outright war, or into the firm peace that we got in TNG. The sacrifice of the Enterprise C (to presumably
save the outpost, although I don't recall if this was ever stated outright on screen) sees the Klingons finally admit the Federation are honorable friends. This is where we got the peace-time of the TNG era.
Alternatively, if the Enterprise C vanishes into the rift and Narendra is instead destroyed by the Romulans, then the uneasy tension escalates again into conflict. Details of what actually happened are sketchy, as there are no survivors other than the Romulans who, naturally, say nothing about what really happened, given it was them
With certain elements in the Federation suspecting that the Klingons had something to do with the disappearance (assumed destruction
) of the Enterprise; the Klingons believing Starfleet had something to do with the destruction of the outpost; and the Romulans sitting back and letting the other two sides fight it out amongst themselves without correcting either incorrect point-of-view, things errupt into the war we see in the Alt Timeline.