According to the War Powers Act.
https://en.wikipedia.org/wiki/War_Powers_Resolution
The War Powers Resolution (also known as the War Powers Resolution of 1973 or the War Powers Act) (50 U.S.C. ch. 33) is a federal law intended to check the U.S. president's power to commit the United States to an armed conflict without the consent of the U.S. Congress. The resolution was adopted in the form of a United States congressional joint resolution. It provides that the president can send the U.S. Armed Forces into action abroad only by declaration of war by Congress, "statutory authorization", or in case of "a national emergency created by attack upon the United States, its territories or possessions, or its armed forces".
The bill was introduced by Clement Zablocki, a Democratic congressman representing Wisconsin's 4th district. The bill had bipartisan support and was co-sponsored by a number of U.S. military veterans.[1] The War Powers Resolution requires the president to notify Congress within 48 hours of committing armed forces to military action and forbids armed forces from remaining for more than 60 days, with a further 30-day withdrawal period, without congressional authorization for use of military force (AUMF) or a declaration of war by the United States. The resolution was passed by two-thirds each of the House and Senate, overriding the veto of President Richard Nixon.
It has been alleged that the War Powers Resolution has been violated in the past. However, Congress has disapproved all such incidents, and no allegations have resulted in successful legal actions taken against a president.[2]
Tons of precedents of presidents ordering military action without authorization from congress, including by Clinton and Obama.
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