The War Powers Resolution isn't what you think it is
One of the most frequently misrepresented federal statutes—often falsely used to justify unconstitutional presidential war powers—is the War Powers Resolution (or Act) (50 U.S.C. §§ 1541-1550).
If only more people would read it.
Contrary to what you may have heard about the War Powers Resolution, it does not allow the president to take military action for any reason for 60-90 days without congressional approval so long as the president notifies Congress within 48 hours.
Section 1541(c) of the War Powers Resolution states clearly:
"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."
Of the three cited authorities, not one indicates a presidential power to take unilateral (without Congress's approval) offensive military action.
The first two authorities allow the president to take offensive military action but only with Congress's express approval (Article I of the Constitution grants Congress the exclusive power to declare war). The third authority allows the president to take defensive military action without Congress's approval in the event of a specific type of national emergency, a sudden unforeseen attack on the United States (happening too quickly for Congress to meet) necessitating immediate action to protect Americans.
It's for this last situation (or for situations in which the president introduces forces into hostilities unlawfully) that the War Powers Resolution provides for the oft-mentioned 48-hour report to Congress (§ 1543) and 60-day (up to 90-day) timeline (§ 1544). If there's an attack in progress on the United States (i.e., currently happening), we expect the president to respond swiftly to neutralize the attack and protect Americans—and then we will hold the president to account.
The Framers of the Constitution agreed at the debates in the federal convention of 1787 that the president should have the "power to repel sudden attacks" but not the power to otherwise introduce forces into hostilities without congressional approval.
The War Powers Resolution does not confer any new authority on the president to take offensive military action without congressional approval—nor could it under our Constitution. It instead checks the president when, as the Framers contemplated, the president introduces our Armed Forces into hostilities to repel a sudden attack.