On January 17, US Secretary of State Rex Tillerson issued a jarring statement about the future of US military involvement in Syria, but his comments barely registered amid the deluge of media coverage surrounding the government shutdown. Tillerson’s speech, however, was no less newsworthy: US forces, he declared, will remain in Syria until Islamic State (ISIS) is totally defeated. The secretary had just revealed the addition of another major campaign in the never-ending Global War on Terror.It’s been nearly two decades since lawmakers passed the 2001 Authorization for the Use of Military Force, the congressional approval on which the anti-terrorism fight is legally founded. Yet the White House continues to wield the vague authorization to expand US military involvement into some of the most volatile places in the world. These operations are being carried out by American military personnel, on behalf of the American people. Their ramifications, including the deaths of thousands of civilians, will span generations. Congress, the branch of government most representative of the nation, has allowed them to continue virtually on autopilot.Passed in the feverish days immediately following September 11, 2001, the AUMF provided the flexibility to pursue the shadowy perpetrators of the attacks around the globe. This authority included using “all necessary and appropriate force,” and it was rapidly put to use on the timeworn battlefields of Afghanistan. But as the years have passed, the authorization has been stretched to justify a widening range of military actions, including some well beyond the scope of the War on Terror as it was initially conceived. With the acquiescence of Congress, the AUMF has evolved into a malleable 60-word justification for forever war.Terrorist organizations are not conventional armies, and the War on Terror wasn’t intended to be a conventional fight. Even as the fledgling US military presence in Afghanistan steadily grew into a lumbering occupation, the Pentagon began opening new fronts across the Middle East, Asia and Africa. Only one – the Iraq War – was congressionally sanctioned under a second AUMF. All the others, dozens of military actions in at least 12 countries, rested at least in part on the legal authority of the initial authorization.As the war expanded, Congress watched as three consecutive presidents launched fresh military endeavors under the 2001 authorization. Tens of thousands of Americans have been killed and wounded, the war’s cost has ballooned into the trillions of dollars, and the list of enemies has broadened to include organizations like ISIS, which didn’t even exist at the time the authorization was authored. The same flexibility that once made the AUMF so effective has now entangled US forces in a region-wide conflict that has no clear borders, no escape route, and no end in sight.Nowhere is this more apparent than in Syria, where roughly 2,000 US military personnel are now based. Devastated by seven years of brutal civil war, the country has devolved into a Hobbesian nucleus of competing sectarian and geopolitical interests. The complexity of the situation is head-spinning: Russian and American planes divide the sky, Iranian proxy forces besiege Sunni villages, jihadists wrestle for dominance in the areas still outside government control. And while a victory for the Assad regime now seems all but certain, the major players are gearing up for a prolonged battle for post-war influence.Wary of being dragged into another protracted counterinsurgency, the US-led coalition has focused on destroying ISIS. It has made considerable progress toward this goal, having reduced much of the physical caliphate to rubble. But this success could come with serious trade-offs. By arming and supporting the Kurdish-dominated Syrian Democratic Forces – fighters Turkey considers terrorists – Washington inadvertently entangled itself in one of the region’s most deep-rooted ethnic struggles. This decision has wedged the US between its main local partner on the ground and a longstanding NATO ally, a reality made all the more perilous by Turkey’s freshly launched military incursion into northern Syria. The Turkish operation not only imperils American forces, it also serves as an example of the sort of nightmare scenarios a prolonged presence in Syria could lead to.Turkish tanks began rumbling across the Syrian border just days after Tillerson’s speech, which featured five US goals for Syria. Tellingly, just one of these directly concerns defeating ISIS. The rest, which include a political transition and degrading Iranian influence, may be important to Syria’s long-term stability, but they have little explicit connection to September 11. This places the current US mission far outside the mandate of the 2001 authorization and signals a continuation of the kind of nation-building efforts the Trump administration vowed to shift away from.Destroying the ISIS caliphate was essential to national security. So is ensuring that it doesn’t return. This will involve targeting the group’s members, denying it sanctuary, and addressing the underlying economic and sectarian issues that fuel its existence. But the scope of this campaign – and the long-term goals espoused by Tillerson – have outgrown the legal base on which it was launched. If the US is committing to another open-ended battle in the anti-terrorism war, lawmakers must stand up and approve the fight.The situation in Syria exposes a simple truth: as long as Congress dodges its constitutional responsibilities, presidents will continue to commit the armed forces to wars without real nationwide support. But it also holds a deeper lesson, one US lawmakers should pay close heed to: when you sign a blank check for war, don’t be surprised when the fight goes on forever.The author has lived in Syria and is currently a graduate student focusing on national security, intelligence and diplomacy at the University of Texas’s LBJ School of Public Affairs. His work has been featured in publications including the Dallas Morning News and Task & Purpose.