The Marine Corps does augment the U.S. Army quite well.
As a DC police patrolman in the mid-70’s, I was partnered with a Marine veteran who exemplified the doctrine, “polite to all, friendly to none.” It was while enjoying an after-shift beer at a local DC watering hole that we were joined by a group of Army officers. In the course of the evening, one of the Army officers started spouting off about how the Army makes the world safe for democracy. My partner quietly responded, “That’s true. But the Marines make the world safe for the Army.”
The silence was deafening.