The U.S. Marine Corps is nothing if not versatile. What explains the Corps' talent for metamorphosis and its ability to take on so many roles and missions was summed up by Marine Lt. Gen. Victor H. "Brute" Krulak, who wrote in 1957, "The United States does not need a Marine Corps." All of its missions could be performed by other services. It is, after all, essentially a "second army" with a small "second air force" attached. To justify its existence in the face of constant naysaying, the Corps has had to take on missions that no one else wanted, and it has had to perform them better than anyone else could. That has led the service to cultivate an unrivaled warrior ethos and a culture that places more emphasis on men and women than on machines. Marines are taught to think of themselves as Marines first and only secondarily as tank drivers or helicopter pilots or infantrymen. This helps to make them unusually adaptable and cohesive, but each major transformation has come with pain. In the 1920s and 1930s, for instance, there was considerable tension between Marines who wanted to emphasize small wars and those who thought the Corps' future lay in amphibious warfare.

http://www.armedforcesjournal.com/2006/03/1813950