To clarify the situation, in the wake of the Vietnam War (another undeclared war), Congress passed the War Powers Resolution of 1973 (50 U.S.C. 1541-1548). This is the current law of the land with regards to the use of US forces in combat by the President. The basic purpose is to define when a President can legally and constitutionally commit forces to combat, and place limitations on their use.
Legal use of US forces in combat:
Section 1541c lays out only three cases when the President can launch forces into combat:
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
Later sections of the law lay down consultation and notification requirements. For example, section 1542 requires prior consultation with Congress, when possible. Section 1543 requires written notification within 48 hours of combat. Section 1544 requires the President to terminate combat within 60 days if Congress has not authorized either the combat action, or provided an additional 60 days.
Many people mistakingly believe that these sections give the President a 60-day blank check. They do not. Nothing in the law relieves the President of meeting sections 1541 and 1542 of the law. The consultation and notification requirements are IN ADDITION TO meeting one of the three conditions listed in 1541c.
What about UN authorization?
Some people believe that a UN Security Council resolution authorizes the President to use force, perhaps counting as statutory authorization under 1541c. This is completely incorrect. Section 1547.a.2 of the law specifically addresses this thought and says:
Authority to introduce United States Armed Forces into hostilities...shall not be inferred...from any treaty heretofore or hereafter ratified unless such treaty is implemented by legislation specifically authorizing the introduction of United States Armed Forces into hostilities or into such situations and stating that it is intended to constitute specific statutory authorization within the meaning of this chapter.
This means unless there is a separate US law, in addition to the treaty, that authorizes that specific combat action, it is NOT authorized under US law, regardless of what the treaty says. There is no US law that gives the President blanket authority to use force if the UN authorizes it.
Is the combat in Libya legal?
No! The President has failed to meet any of the three conditions laid out in 1541c. It really is that plain and simple. Congress can make it legal by authorizing the action within 60 days. Until that happens, the President is outside the law.
Will Congress authorize the action? Will Congress call the President to account for violating the law? Some have tried without much support.