There are 2 distinct movement phases. The Friendly Movement phase and the Moving into Enemy Territory Phase. The Friendly phase is before the Enemy phase, so if you guess right, you can move an army into a previously undefended position where you opponent thinks you are weak and end up defending it.

The main issue is the fact that the magic phase is before both phases, so you can lay on the magical pain with the full knowledge of where armies are if you have proper scouts/intel.