United States territorial acquisitions
The Panama Canal Zone was an unorganized US territory located within the Republic of Panama. It was established under the Hay–Bunau-Varilla Treaty in 1903 and disestablished in 1979 under the Torrijos–Carter Treaties. Panama gained full control over the Panama Canal in 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.