Outgoing Defense Secretary Robert M. Gates, who cut major weapons programs during 4½ years at the Pentagon, is warning against “hollowing out” the U.S. military through unwise spending cuts planned by the Obama administration.

“I am determined that we not repeat the mistakes of the past, where budget targets were met mostly by taking a percentage off the top,” Mr. Gates said in a speech to a conservative think tank Tuesday.

“That kind of salami-slicing approach preserves overhead and maintains force structure on paper, but results in a hollowing out of the force from a lack of proper training, maintenance and equipment - and manpower”...