The Marines are the Navy's army (small A), the amphibious ground force that launches from the sea and can land on just about any beach in the world. They use amphibious landing craft held inside Navy amphibious assault ships. The Marines also have their own airwing that provides close air support for their ground troops.
The marines are, Considered the best marksmen set in the world (All marines are trained marksmen). They deal with a significant amount of battles, (one of which was Iwo Jima). and have been a part of the military since their founding at tuns tavern during the revolutionary war.
They serve our country: they "fight our nation's battles, on the land and on the sea". If need be, they die (and many HAVE died) for this country. And, they are, arguably, the most enthusiastic arm of the United States military.
KILL. The President uses the military as a tool to kill as many people and wreak as much destruction as necessary on an opposing country who has gone too far, to convince them to return to more politically sensible methods. The military is an extreme political response used when reasonable actions have failed.