He could have ordered a military action, but actually Congress ARE the only ones who can declare war. The Constitution gave that power to the legislative branch for a reason:
The framers of the Constitution—reluctant to concentrate too much influence in the hands of too few—denied the office of the President the authority to go to war unilaterally. If America was going to survive as a republic, they reasoned, declarations of war required careful debate in open forums among the public’s representatives.
One of the things the framers found objectionable with monarchies was the ability of a king to declare war when it wasn't in the best interests of their people.
Read. Learn.
https://history.house.gov/Institution/Origins-Development/War-Powers/