BT is simply an example where this problem is an extreme - I'm talking about the game as a whole.
And yes, you can be at a state of declared war ALL of the time, where and troopers from opposing parties who happen upon one another fight. War doesn't have to mean "everyone glom their troops into 10k cs groups and march together"
This makes me wonder if you play in Melhed by any chance? :p
Anyway, BT is just a little different as we often have to fight large invader forces together, trying to keep humanity as a whole alive. This is now true in a lesser extent as before though. Nowadays we've got realms siding with invader-forces left and right, but if you declare on BT that you truly want to the invaders to win in the end, you're sure to be shunned by most - probably all - realms.
My point being that while there are always conflicts looming on BT, there always remains the knowledge that one day you'll have to work together with your enemy if you are to survive. Everybody has experienced the death and destruction of the invasion and it is considered improper to attack a realm while it is still recovering. Thalmarkin for example had a total of 4 downsized RCs, level 1 walls in the capital and about 10,000 peasants left by the end of the invasion. There's no doubt that Fronen or Melhed could have attacked us (Melhed often had a bigger army than our own in our lands to help us against the hordes) yet they didn't for that reason.
It is also the reason that it has taken this long for war to break out again. But I assure you that it will be constant fighting from her on until the next invasion. I've never known it different, and I've lived through all invasions. Never in a realm that died and never in a realm that allied with the invaders I might add. I hope I didn't jinx it now... :p