I think the Germany comparison WWI-WWII isn't the right one to be making here.
Praxis/Khitomer is 2293, Narendra III is 2344, 51 years. The more apt Earth comparison here would be Japan 1945-1990s. After being decimated in WWII, including the bombing of two key industrial cities, Japan had decades of rebuilding ahead of them. However, partially due to the rebuilding and aid provided to them after the war, they came back stronger than ever. It's conceivable that in some alternate timeline, something catastrophic may have caused Japan and the United States to fight again. Maybe the Russians attack a key Japanese outpost in the Pacific, and a lone US battleship happens to be nearby. Their actions may either cause sympathy or hatred for the United States by Japan.
So, going back to TUC/YE, we have a military society that is wreaked with devastation following the loss of an important industrial center with the destruction of Praxis, and we have the UFP providing humanitarian aid after a peace agreement is reached. 50 years later, Kronos is rebuilt but tensions are mounting, because the Klingons don't like being subservient to the UFP and their assistance. Romulans attack Narendra III, and a lone Starfleet vessel is in the area to provide assistance.
The only thing this doesn't resolve is why in YE they're in negotiations for peace again (peace should already be achieved after Khitomer), but I'm willing to let this one slide. The rest is conceivable.
Praxis/Khitomer is 2293, Narendra III is 2344, 51 years. The more apt Earth comparison here would be Japan 1945-1990s. After being decimated in WWII, including the bombing of two key industrial cities, Japan had decades of rebuilding ahead of them. However, partially due to the rebuilding and aid provided to them after the war, they came back stronger than ever. It's conceivable that in some alternate timeline, something catastrophic may have caused Japan and the United States to fight again. Maybe the Russians attack a key Japanese outpost in the Pacific, and a lone US battleship happens to be nearby. Their actions may either cause sympathy or hatred for the United States by Japan.
So, going back to TUC/YE, we have a military society that is wreaked with devastation following the loss of an important industrial center with the destruction of Praxis, and we have the UFP providing humanitarian aid after a peace agreement is reached. 50 years later, Kronos is rebuilt but tensions are mounting, because the Klingons don't like being subservient to the UFP and their assistance. Romulans attack Narendra III, and a lone Starfleet vessel is in the area to provide assistance.
The only thing this doesn't resolve is why in YE they're in negotiations for peace again (peace should already be achieved after Khitomer), but I'm willing to let this one slide. The rest is conceivable.