Gen. Maxwell Thurman, commander of the U.S. Southern Command and the field commander in charge of the 1989 invasion of Panama, is retiring, the Defense Department announced yesterday.

Thurman, 59, who learned that he had leukemia in July, is undergoing chemotherapy treatment at Johns Hopkins Oncology Center in Baltimore.

The Pentagon said Lt. Gen. George A. Joulwan, 50, currently commanding general of the 5th Corps of the U.S. Army in Europe and the Seventh Army, has been nominated by President Bush to replace Thurman and to be promoted to the rank of general.

Thurman has been commander of the U.S. Southern Command, based at Quarry Heights, Panama, since September 1989. He played a central role in drafting the plan for the December 1989 invasion.

The four-star general had been scheduled to take mandatory retirement in August 1989 but Defense Secretary Richard B. Cheney insisted that he take over the Southern Command to assist in the administration's struggle with Gen. Manuel Noriega's regime in Panama.

A native of High Point, N.C., and a graduate of North Carolina State University, Thurman held numerous key Army posts, including vice chief of staff and commanding general of the U.S. Army Training and Doctrine Command at Fort Monroe before taking the top spot at Southern Command.

Cheney also announced yesterday that Adm. Huntington Hardisty, commander in chief of the U.S. Pacific Command, is retiring after 38 years of active service. He is to be succeeded by Adm. Charles Larson, currently commander in chief of the U.S. Pacific Fleet.