Having converted spies, getting hold of the enemy’s spies and using them for our own purposes.
If you can catch and turn the enemies spies, you then have a double-agent who you can get to send back false information and to tell you critical information, in particular about other spies (although spy networks are often organized to prevent such information being passed on).
You may be able to persuade the spy to turn, or you may need to use a more coercive approach. The threat of harsh punishment may be sufficient for some.
Beware of enemy spies pretending to be double agents or acting as triple agents. This is a murky world and you can never fully trust double agents.