The United Canadian and American States Marine Corps (USMC) is the naval infantry of the UCAS Navy and a branch of the UCAS Armed Forces. Unlike other states' naval infantry (namely, Russia's), the Marine Corps is effectively a military-within-a-military that possesses its own air and naval assets to deploy forces without the direct support of any other branch.
When the CAS seceded from the UCAS in 2034, those Marines who left to join the CAS and form the CAS Marine Corps (CSMC) couldn't simply cut all ties with the UCAS Marine Corps. While they are separate military branches from separate nations, the USMC and CSMC continue to cross-train and exchange personnel. Confederation Marines regularly train at Marine Corps Base Quantico, among others, alongside UCAS Marines.
The Marine Corps considers itself an elite force of its own, but there are special purpose units within the Marine Corps that are more appropriately considered to be elite military forces: Fleet Anti-Terrorism Teams (FAST) and USMC Special Forces (MCSF).