"A sunset clause precommits Congress and the President to revisit the nature and scope of the very diffuse war against Islamic terrorists on a regular basis. It pressures the President on a regular basis to explain the nature of the conflict and the reasons why it must continue (and how), and it pressures Congress to exercise its constitutional and democratic responsibilities to deliberate about and vote on (or at least face) the issue. A sunset clause poses risks. The President could argue after three years that he can continue to fight the Islamic State under Article II without Congress’s imprimatur. But while this argument might be available, it is more of a legal stretch, and certainly politically riskier, for the president to justify the entire conflict on the basis of Article II than it is for him to use Article II to extend the war a bit beyond what Congress authorized (either geographically or in terms of ground troops). There is also the danger that Congress will not renew the authorization, or not renew it to the President’s liking, thus forcing the President into an awkward Article II posture or into narrowing the conflict further than he would like. I doubt any of these outcomes will occur, as it should not be hard for the next President to make the case for renewing authorizations for war if such authorizations are plausibly warranted. But in any event, as a nation we should not shy away from these possible consequences of democratic deliberation on a vital question like war.
In short, a sunset clause is the one provision that a Congress seeking to reassert its constitutional prerogatives should insist on in a new AUMF." Goldsmith in Lawfare
The great danger that we face in the US is that of an unending war against any or all forces that a president can describe as being of hostile intent and capable of carrying out kinetic attacks on the US or powers friendly to the US.
The AUMFs have become the functional equivalents of declarations of war. Under their authority the president and commander in chief can wage war with little restriction on his Second Amendment powers as Commander in Chief of the armed forces.
Unless these powers are limited in some meaningful way we are watching a transformation from Republic to Principate. pl