to reduce the size of your army, navy etc, and the number of your weapons [≠ arm]:
Getting the rebels to disarm will not be easy.
to take away someone's weapons [≠ arm]:
Captured soldiers were disarmed and put into camps.
to make someone feel less angry or disapproving of you, and more friendly [↪ disarming]:
His tact and political skills disarmed his critics.
to take the explosives out of a bomb, missile etc