Rearmament is the name of the whole process or plan of getting weapons again, or of giving a country new weapons. History classes use it for the years when a country builds its army back up — German rearmament in the 1930s is the example many books still reach for. Same idea as rearm, but here you are naming the program, the stretch of time, the government plan, not one day’s action. It is almost always about countries and armies, not a person buying one gun. Formal, newsy, a little cold. Though actually, writers pick it when they want you to see a direction: not just “they bought weapons,” but “they are going back to being armed, on purpose, as a line of policy.”