History of the Panama Canal
During World War II, the canal proved a vital part of the U.S. military strategy, allowing ships to transfer easily between the Atlantic and Pacific. Politically, the Canal remained a territory of the United States until 1977, when the Torrijos–Carter Treaties began the process of transferring territorial control of the Panama Canal Zone to Panama, a process completed on 31 December 1999.
Territories of the United States on stamps
The U.S. Postal Department also used general issue stamps with the territorial name overprinted, such as PUERTO RICO, CUBA, GUAM, PHILIPPINES or CANAL ZONE. Cuba was granted independence, Canal Zone returned to Panama. 
Alternatively, the U.S. issued a 1932 'United States of America' stamp for 18 centavos in the Philippines, 1930 stamps for use in Guam only designated 'Guam Guard Mail, or 1952 'Ryukyus' stamp for 100 yen in Okinawa. The Philippines was granted independence, the Ryukyus were returned to Japan. 
Panama Canal has been featured on several U. S. commemorative stamps, as well as on Panama's own postage. The territory was returned to Panama.