An army is a group of ground soldiers under the command of a general. Armies were the main method of combat between kingdoms in the pre-modern era.
Much of military strategy in the immediately pre-modern era centered around the question of whether or not it is preferable to maintain a "standing army" - that is, a group of soldiers ready to be "mobilized" at a moment's notice to fight for the country, functioning additionally as deterrence. Standing armies are exorbitantly expensive to maintain. Now, most countries maintain a small standing army, but nuclear powers rely on nuclear warheads to maintain deterrence in large part.