WASHINGTON (AP) _ John Hamre, the deputy secretary of defense, is resigning to become head of a major Washington-based think tank, a senior defense official said today.

Rudy de Leon, the undersecretary of defense for personnel and readiness, will be nominated to replace Hamre in the No. 2 Pentagon post, the official said, speaking on condition of anonymity. Like Hamre, de Leon came to the Pentagon when Les Aspin became President Clinton's first defense secretary in 1993.

De Leon was undersecretary of the Air Force from 1994-97. Before becoming Aspin's special assistant in 1993, de Leon was staff director for the House Armed Services Committee, which Aspin had chaired.

The Pentagon planned to announce today that Hamre would leave at the end of March and become head of the Center for Strategic and International Studies, the official said. Hamre has held the No. 2 job since July 1997. Before that he was the Pentagon's comptroller for four years and he spent 10 years as a staff member of the Senate Armed Services Committee.

Defense Secretary William Cohen accepted Hamre's resignation last week, and Hamre informed his staff today.

Hamre took the lead role in preparing the Defense Department's far-flung computer networks for the Y2K rollover.

In his new job at the Center for Strategic and International Studies, Hamre will oversee one of the nation's most prominent public policy research institutions. Founded in 1962, the center's current president is David Abshire, a former U.S. ambassador to the North Atlantic Treaty Organization.