The computer player's artificial intelligence has undergone some radical improvements since the original KKND. The AI can now calculate the size of the force it requires to win a battle, and groups and manoeuvrers its units in a more coherent way, ensuring that attacks on bases, for instance, are more effective than they used to be.
The AI spends time and energy mapping out the terrain and scouting out the location of your forces so it can plan the best attack route from a number of choices. It breaks down the map into a series of zones and calculates your units' threat in each zone. The AI will not take on a superior force if it does not have to - it will plan a more circuitous route.
The opponent AI knows when to face you, and also when to turn and run. In the original KKND it was possible to coax an AI's army into the open with a few attacking troops before the AI had assembled a coherent force. In KKND2, the AI is always prepared to play the waiting game, and won't attack or be lured into the open before it's ready to face a full-on assault.