50 USC § 1541 - WAR POWERS RESOLUTION, Purpose and policy
(c) Presidential executive power as Commander-in-Chief; limitation
The constitutional powers of the President as Commander-in-Chief to introduce United States Armed Forces into hostilities, or into situations where imminent involvement in hostilities is clearly indicated by the circumstances, are exercised only pursuant to (1) a declaration of war, (2) specific statutory authorization, or (3) a national emergency created by attack upon the United States, its territories or possessions, or its armed forces.
https://www.law.cornell.edu/uscode/text/50/chapter-33
NSPD-17 / HSPD 4 [unclassified version]:
National Strategy to Combat Weapons of Mass Destruction
December 2002
https://fas.org/irp/offdocs/nspd/nspd-17.html
Codified by Congress making its statutory law. Mostly still classified.