United States Department of Defense | 1,511,218 followers on LinkedIn. The mission of the Department of Defense is to provide military forces necessary to protect the security of our country. The U.S. military defends the homeland, deters adversaries, and builds security around the world by projecting U.S. influence and working with allies and partners. In case deterrence fails, the U.S. military is prepared to fight and win decisively against any adversary.