WASHINGTON — THE United States is fighting an unauthorized war. Over the past 19 months, American forces have launched more than 8,800 strikes against the Islamic State in Iraq and Syria and hit the group’s affiliate in Libya. The United States continues aerial assaults against Al Qaeda in the Arabian Peninsula in Yemen, is going after militants in Afghanistan and Pakistan, and killed more than 150 suspected Shabab fighters in Somalia just last month.  This war isn’t limited to drone strikes or aerial bombings. It includes Special Operations forces in Iraq, Syria and Afghanistan — and possibly elsewhere. This past weekend, President Obama announced that he would send an additional 250 such troops to Syria.  The primary legal authority for these strikes and deployments comes from the 60-word Authorization for Use of Military Force, passed almost a decade and a half ago. In the days after Sept. 11, 2001, President George W. Bush asked for an open-ended authorization to fight all future acts of terrorism. Wisely, Congress rejected that request, though it did give the president authority to use force against those responsible for the Sept. 11 attacks, Al Qaeda, and those that harbored them, the Taliban.  Today, the Taliban no longer rules Afghanistan, Osama bin Laden has been killed and the other key participants in the Sept. 11 attacks are either locked up or dead. But the old authorization lives on.  The Obama administration has reinterpreted the 2001 authorization to fight not just those entities referred to in the law, but their “associated forces” and successor organizations as well. Put bluntly, the United States is relying on an authorization to fight those responsible for Sept. 11 to wage war against groups that had nothing to do with those attacks and, in some cases, didn’t even exist at the time. This expansive legal interpretation empowers future presidents in dangerous ways.  Mr. Obama, to his credit, recognizes the problem. In May 2013, he said he would “refine, and ultimately repeal” the 2001 mandate. Last February, he proposed a new authorization specifically aimed at the Islamic State. But his proposal was rightly critiqued by just about everyone. Among other problems, it left the 2001 authorization in place, meaning that the proposed authorization would merely add to, rather than replace, the existing authority. Members of Congress responded with a range of alternatives, but none made it into law.  With nine months left in office, the administration should now revive these discussions. Mr. Obama has long warned of the hazards of unbounded war, but the approach his administration is taking sets the precedent for just that. While the groups that the United States is attacking must have some nexus to Al Qaeda and the Taliban, the limits are slippery. The Islamic State, a successor to Al Qaeda, is deemed covered by the 2001 authorization. What about the Islamic State’s successors? Their successors?  Such an expansive reading of the 2001 authorization has now been normalized, in large part endorsed by Congress and, to some extent, the courts.  Future administrations could use this authority as they saw fit. They might act in a circumscribed manner, precisely and carefully picking targets. Or they might seek to “carpet bomb” the enemy, as the Republican presidential candidate Ted Cruz has said he would do. As it now stands, the next president could do so against an array of yet-unspecified groups.  This is worrisome. There is a good reason the founders gave Congress the authority to declare war and the president the authority to wage it. The decision to go to war — even when carried out remotely from the air with minimal risk to Americans — is simply too important to entrust to a single branch of government.  To be fair, Mr. Obama is not the only one to blame. Congress has abdicated its role, choosing to defer to the executive branch rather than take on a potentially controversial issue. And it would be naïve to think that a Congress that won’t even consider the president’s Supreme Court nominee is going to give him a “victory” by approving a war authorization.  But that doesn’t mean the president shouldn’t try. Even if he doesn’t succeed, he can lay the intellectual and political groundwork for a new authorization, making it that much easier for the next administration to push forward. He should propose an authorization to use force against the groups the United States is actually fighting and insist on a sunset provision so that Congress is forced to remain engaged.  A new war authorization isn’t likely to change the facts on the ground. Anything Mr. Obama proposes is going to allow him to use the kind of force he has already deemed necessary. But it still matters for reasons of good governance, protecting the balance of power between Congress and the executive branch, and ensuring that when the nation takes the extraordinary step of going to war it does so on behalf of and with the consent of the people. This is a legacy issue that Mr. Obama should now address.  Jennifer Daskal, an assistant professor at American University Washington College of Law, was counsel to the assistant attorney general for national security in the Justice Department from 2009 to 2011.  A version of this op-ed appears in print on April 27, 2016, on page A23 of the New York edition with the headline: Obama’s Last Chance to End the ‘Forever War’. Today's Paper|Subscribe
The Obama administration has been fighting a semi-legal war for almost eight years. That’s not a power he should leave for the next president.