United States Navy (usually, but can be any nation's fleet). The Navy is the sea-based branch of the military, but through the use of
aircraft carriers they have taken over much of what the Air Force used to do. Because of this they are possibly the most prominent and widely-used of all the branches of the U.S. military (same goes for most other nations too).
Tactically, their primary focus is blue-water operations such as fleet and subarmine warfare, but since the Cold War this role has been de-emphasized as most countries with any respectable naval forces arent dumb enough to piss off the USA.
Their role in combat operations will likely only continue to increase as everyone loves a "clean" air war. However, because they do not have a true ground force anymore they are not self-sufficient and must work very closely with the Army/Marines during combat operations.