Other forms: warships
A warship is a military sea vessel that's usually armed with weapons. A country's navy uses warships in battles or to transport troops.
A battleship is one specific kind of warship, a very large one that's outfitted with weapons and heavily defended against attack. All warships are prepared for war (which is where they get their name), and most of them are controlled by the naval forces of a nation. Many countries move their warships into particular regions to exert political force on adversaries, or send them to protect civilian or cargo ships from pirate attacks.