CAMP PENDLETON, Calif. — Navy Secretary Ray Mabus had a simple message for 1,500 Marines and sailors: The decision to let women compete for all military combat positions is as irreversible as earlier edicts to integrate blacks and allow gays and lesbians to openly serve.

It was Mabus’ third visit to a major Marine Corps base to explain the issue to rank-and-file audiences since Defense Secretary Ash Carter said in December that all combat positions would be open to women.

Mabus repeatedly emphasized that standards won’t be lowered.

“Marines, we’re past the decision now. The secretary of defense has made the decision. Now we’re into implementing,” he said Tuesday at Camp Pendleton in California.

Marine Corps leaders had sought to keep certain infantry and combat jobs closed to women, citing studies showing combined-gender units are not as effective as male-only units. Carter, backed by Mabus, overruled them.

Since December, the military services have put together plans outlining how they will integrate women into male-only units.

Marines who sat cross-legged around Mabus on a large concrete surface used for ceremonies didn’t object to the change when the secretary invited questions. Some who volunteered to speak with reporters said any resistance might come from older Marines.