Bush authorized the eavesdropping of U.S. citizens and visitors in 2002, and the policy was then carried out by the National Security Agency.
Bush said Sunday at the Brooke Army Medical Center in San Antonio, it was important "to find out what the enemy's thinking."
Former deputy attorney general James B. Comey refused to sign onto the program in March 2004, prompting administration officials to visit then-Attorney General John Ashcroft in the hospital to get his approval.
Comey's reluctance caused the program to be halted temporarily.
Senate Judiciary Committee Chairman Arlen Specter, R-Pa., said he will conduct hearings on the program when the new session of congress begins.