The President may use the armed forces pursuant to:
a declaration of war by Congress;
specific statutory authorization; or
a national emergency created by an attack upon the United States, its territories or possessions, or its armed forces.
§ 1542: Consultation of Congress
The President must consult Congress before introducing military forces into hostilities—or where hostilities are imminent. He must regularly consult, additionally, after doing so.
§ 1543: Reporting Requirements
In the absence of a Congressional Declaration of War, after introducing forces into hostilities, the President must submit a report to the Speaker of the House within 48 hours. This report must outline:
The circumstances necessitating the use of the armed forces;
The constitutional and legislative authority for doing so; and
the estimated scope and duration of the hostilities/involvement.
Must be reassessed every six months.
The President must also provide other requested information and status reports.
§ 1544: Congressional Action
The President shall terminate the use of the armed forces within 60 days after submitting a report, unless Congress:
declares war or enacts specific authorization;
extends the 60-day period by law; or
is physically unable to meet due to an armed attack on the United States.
There are no formal definitions of "hostilities," "imminent," or "consultation."
Therefore, the executive's determination typically controls here.