A United States federal law providing that the President can send U.S. armed forces into action abroad only by authorization of Congress or if the United States is already under attack or serious threat. From Wikipedia
The War Powers Resolution of 1973, the United States Congress joint resolution providing that the President
To limit the power of the president to commit American troops to military action without authorization from Congress.
National.
To limit the the President's war-making powers.
Each state does not have the power to declare war. Only Congress can declare war.