I imagine playing Combat as War for the monsters can be a lot of fun, but I think it needs quite a lot of prep by the DM as you need to create systems for the PCs to interact with to learn information about the enemy, so that they too can approach Combat as War. You also need to make sure they use those systems.
What I mean, is if you want to run the monsters in a way that they are trying their hardest to leverage a 10:1 advantage over the PCs, you need to give the PCs the tools to start tracking the movements of the enemy (like scouts or spies), their supply routes, and the lay of the land. PCs need to be able to gather information that will allow them to outsmart the enemy - strike at weak points, and so on so that they too can leverage a 10:1 advantage through clever play.
This is not an insurmountable amount of extra prep, and it could be really rewarding if this type of play appeals to your players. The reason I say this prep is required is because the last thing you want is a GOTCHA to occur. If the PCs walked blindly into an area to be suddenly confronted by the enemy with a 10:1 advantage over them, I imagine it won't be a positive gaming experience for them. They need to know that the enemy are trying to do this, and they need to be able to do the same.
What it does run the risk of, however, is making combat encounters dull. The example I will use is the Total War computer games. For anyone who has not played these games, there is essentially 2 systems at play. The campaign map and the battles that result from it. At the start of every Total War game, the battles are very exciting because neither you nor the opposition can leverage a great advantage, meaning the battles are closely fought and their outcomes very impactful. As the game progresses and the winning factions are able to muster greater and greater resources, there comes a tipping point where the battles become irrelevant. The stakes are not impactful as the faction with greater resources can weather the loss and grind out tough opposition through attrition. At this point, the player usually starts automatically resolving the battle rather than playing through them. There is a risk the same thing could happen to a Combat as War D&D campaign where the leveraging of advantages makes the battles forgone conclusions.