The United States Marine Corps was established as the Continental Marines on November 10, 1775. From its founding through the beginning of the 20th century, Marines were primarily associated with ships detachments and served as provisional forces for expeditionary service ashore.
Through the 20th century to present day, the Marine Corps has been involved with conventional wars (World Wars I and II, Korea, Desert Storm, etc.) but also hybrid wars, among them Vietnam, Iraq and Afghanistan.

Semper Fidelis distinguishes the Marine Corps bond from any other. It goes beyond teamwork—it is a brotherhood that can always be counted on. Latin for "always faithful," Semper Fidelis became the Marine Corps motto in 1883. It guides Marines to remain faithful to the mission at hand, to each other, to the Corps and to country, no matter what. Becoming a Marine is a transformation that cannot be undone, and Semper Fidelis is a permanent reminder of that. Once made, a Marine will forever live by the ethics and values of the Corps. In addition to Semper Fidelis, Marine Corps Officers also embrace the phrase Ductos Exemplo, "to lead by example," the motto of Officer Candidates School (OCS). Instructors look for candidates who display self-reliance, discipline and responsibility. Desire and motivation to lead Marines are deciding factors in an officer's success.

Excerpted from the Marine Corps’ officiate web site.