rearmament
Refers to the act of rebuilding or strengthening a nation's military forces, often after a period of disarmament or restriction.
After the war, Japan chose the path of rearmament.
That country is rapidly proceeding with rearmament.
武装 means 'arming' or 'being armed', often referring to equipping with weapons, whereas 再軍備 is a broader political/military concept of rebuilding a nation's military capability.
Compound of 再 (sai, 're-') and 軍備 (gunbi, 'armaments, military preparedness').