It's not good when people are still upset over perceived slights that happened 5+ RL years ago. Most players in the game haven't been here that long, so basing your foreign policy on events that old is just going to upset and confuse them.
While that is true, there is also the "We have always been at war with Eurasia" mentality at play. Even if the actual perceived slight is from so long ago that few characters alive then actually remember it, the resulting dislike between realms can take deep roots. It would be bad RP to suddenly become friends with a realm you've hated since you were born, even if you have no other reason to hate them than because it was the cool thing to do where you're from.
