Having inward spies, making use of officials of the enemy.
A good spy, if you can turn him or her, is one who is trusted and who works within the enemy forces. Best of all is when they have access to plans and can overhear important discussions.
Many people working in wars are co-opted into their position. Administrators, especially are not natural warriors and may resent being held under strict command.
Inward spies may be gained by simple payment or, better, their simmering resentment may be amplified so they decide to side with you. This can be especially powerful if they believe you will win the war (so persuade them of such).