Article II, Section 2 of the Constitution states that "the President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual service of the United States." Thus, the President is commander of all US troops in every branch of the US military - in the air, on the ground or on the sea.
The limits of the powers of the President as Commander and Chief have been extensively debated. Article I, Section 8 of the Constitution clearly gives Congress the power "to declare war." Nevertheless, Presidents from Adams on have sent American troops into combat without Congressional approval. Two early examples are President Adams in the undeclared war with France (1798-1800); and Jefferson, who had previously favored limitations on presidential powers, in the TripolitanWar (1801-1805). In this century , President Roosevelt ordered the American Navy to open fire on Nazi U boats on sight, without Congressional approval. In addition, President Truman committed US troops to “police action” in Vietnam. All subsequent US commitmens in Vietnam were also made without any explicit declaration of war.
The only enabling legislation was the Gulf of Tonkin Resolution (1964). However, Presidents have always taken the view that, by allocating funds for military actions, Congress was expressing its approval of the actions. The disputes over the use of military force in Vietnam led to the passage of the War Powers Resolution (1973). This act was designed to limit the actions of President in an undeclared war. The major provisions require the President to notify Congress within 48 hours of the introduction of American forces into harms way. If Congress does not approve the use of force within 60 days, the President must withdraw the forces. Since the passage of the law, Presidents have followed the reporting provisions of the law while asserting that they would not be bound by any restriction imposed by the law.