Well, your enemy is often your greatest opponent. Despite flaming, fighting, and general dislike, over time the flaming numbs down (an example is how this thread went, from flame to stats to nothing). After that, you begin to build a respect for the skill of your enemies, and even a slight interest in their habits and how they work. The more you fight, the more you realize that they are a good tribe (if they hold you off, that is). In this case, the tribes were so evenly matched, with the pendulum swinging both ways, that a great respect came up.
Not only that, but you have to realize that the war wasn't going anywhere. The war was slowing both tribes down, but even with a ceasefire we knew that the tribes would lock horns again later on, simply because of territory and the need for expansion. One might say Ad Inf would change that, but it would only prolong the conflict and make it that much bigger.
And lastly, you have to realize that because the war was so draining and time wasting, ORC and BANG! were being left behind in the dust next to the growth of =HRV= and C2. They had become superpowers compared to the rest of the world, and therefore one has to realize that the only way to come back up in the ranks and not lose prominence is the merge.