Can Trump attack Iran without Congress’ approval? His administration is building the case.

If the prospect of President Trump sending 10,000 troops to confront Iran didn’t concern you enough, consider this: He might do so without even asking Congress, and it’s not clear there would be any way to stop him.

Can he do that? A quick glance of the Constitution—which expressly grants Congress the power “to declare War”—may lead you to an emphatic “no.” But quietly, the Trump Administration has been building the case that an old, yet seemingly immortal, statute provides it with all the legal authority it needs.

That infamous statute, the 2001 Authorization for the Use of Military Force (AUMF), has a long and troubled history. When it was passed 18 years ago, it is unlikely that anyone believed it would still be around today, or that it would be even considered as a justification for striking Iran. But under Presidents Bush and Obama, the 2001 AUMF was warped into perhaps the single greatest source of presidential power over foreign affairs in modern history. And there are indications that President Trump may take it to the next level.

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Following the September 11 attacks, Congress immediately began considering legislation to authorize force against those responsible. Initially, President Bush asked Congress for the broad authority “to deter and preempt any future acts of terrorism or aggression against the United States.” Many members of Congress, however, objected to this potentially endless authority, and the language was promptly removed.

What Congress eventually agreed-upon was far more limited, at least theoretically. The legislation that was ultimately signed by President Bush authorized force only against those who “planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001 or harbored such organizations or persons.” At the time, this was widely understood to apply to al-Qaeda and the Taliban only.

Over the course of the next several years, however, Presidents Bush and Obama adopted legal theories expanding the 2001 AUMF to cover powers and groups far beyond what was intended. For example, Bush and Obama claimed that the AUMF not only authorized “force” against covered groups, but also authorized the ability to detain members of those groups. The courts, and eventually Congress, rubber-stamped this new interpretation, which provided the justification for detention facilities such as the one at Guantanamo Bay.

Perhaps most importantly, the Bush and Obama Administrations argued that the AUMF applied to “associated forces” of al-Qaeda and the Taliban. A 2016 report issued by the White House defined an associate force as one that: (1) fights alongside al-Qaeda and (2) is engaged in “hostilities” against the U.S. or its partners. This, of course, signaled a massive expansion of the president’s powers under the 2001 AUMF, and would eventually be cited at least 30 times to cover deployment of troops to ten countries.

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In 2014, a major development occurred in the fight against al-Qaeda: a new, distinct group—the Islamic State—split off from al-Qaeda and began planning attacks against U.S. forces. President Obama soon announced that the U.S. was engaged in direct military confrontations with ISIS, but a problem arose: Did the President need explicit congressional authorization to fight the new group, or did such authorization already exist?

The Obama Administration said the latter, setting the stage for perhaps the most controversial and drastic expansion of executive power under the 2001 AUMF. The Obama Administration argued that the U.S. not only had the power under the 2001 AUMF to fight ISIS, but that such authorization existed since 2004, when ISIS’s founder, Abu Mu’sab al-Zarqawi, began planning attacks in Iraq. The connection between al-Zarqawi and Osama Bin Laden is a subject of ongoing debate—their relationship was far from smooth, al-Zarqawi was killed in 2006, and their groups split in 2014—but the Obama Administration argued that the links, however slight, brought ISIS within the AUMF.

In addition, the Obama Administration claimed that subsequent measures by Congress—such as appropriating funds for the fight against ISIS—provided evidence that the AUMF indeed covered the Islamic State. But many members of Congress vigorously dispute that idea, and objected to the constant expansion of power under the 2001 AUMF. As Illinois Senator Dick Durbin put it in 2013, “None of us, not one who voted for it, could have envisioned we were voting for the longest war in American history or that we were about to give future presidents the authority to fight terrorism as far flung as Yemen and Somalia.”

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Earlier this year, a Washington Times report, citing sources within Congress and the Trump Administration, floated a new theory: that Iran and its proxies can be considered “associated forces” of al-Qaeda, and thus falls within the 2001 AUMF. Coupled with the Trump Administration’s recent claims connecting al-Qaeda and Iran, this may signal the newest frontier in the seemingly endless life of the 2001 AUMF.

To be sure, claims connecting Iran to al-Qaeda are nothing new. During the Obama Administration, the State Department’s annual “Country Reports on Terrorism” noted, “Iran previously allowed [al-Qaeda] facilitators to operate a core facilitation pipeline through Iran since at least 2009, enabling [al-Qaeda] to move funds and fighters to South Asia and Syria.” When Trump took office, however, the State Department removed the words “previously allowed,” placing the ties squarely in the present. This intelligence will likely place a key role should the Trump Administration decide to expand the 2001 AUMF.

Recently, the claims regarding Iran and al-Qaeda have begun to ramp up. The Trump Administration recently claimed that Iran is building up its proxies in Iraq, potentially placing Iranian hostilities directly alongside al-Qaeda (a key British general rejected this claim, and the Administration has offered no evidence to support it). In April, Secretary of State Mike Pompeo told a Senate committee, “There is no doubt there is a connection between the Islamic Republic of Iran and al-Qaeda. Period, full stop.” Pompeo later said plainly that President Trump does not need Congressional authorization to strike Iran.  And earlier this week, the top U.S. commander in the Middle East warned that an attack by the Iranians is imminent.

As Steve Vladeck notes in Just Security, there are two glaring problems with extending the 2001 AUMF to cover Iran. First, the Trump Administration has failed to adequately draw the factual connection between Iran and al-Qaeda; thus far, it is not at all clear that Iran sufficiently “harbors” members of al-Qaeda to trigger the AUMF. Second, even if the Trump Administration could make that case, the AUMF itself is not clear on whether the “harboring” had to take place prior to the 9/11 attacks, or if it applies indefinitely. In any case, there is no clear avenue for challenging (let alone stopping) the Trump Administration’s decision should it strike Iran without Congressional authorization.

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The 2001 AUMF, which will celebrate its 18th birthday in September, is the longest congressional authorization for the use of military force in U.S. history. But the sole member of Congress to vote against it, Barbara Lee (D-CA), wants to change that.

A bill sponsored by Lee, currently pending in the House, would repeal the 2001 AUMF. The bill, which criticizes the 2001 AUMF as “inconsistent with the authority of Congress to declare war,” has 54 co-sponsors, including some Republicans. Even if passed, the repeal would not take effect for several months, and of course, President Trump can simply veto it. In addition, with all of the military operations currently tied to the 2001 AUMF, it is unlikely that members of Congress will support a full repeal without leaving something in its place.

Thus, should any further extension of the AUMF occur under President Trump, it is unlikely that Congress could do anything in time to stop it. What about the courts? Typically, challenges to the president’s authority under the AUMF reach the courts only through a rare proceeding called habeas corpus, through which a detained individual can challenge the legal authority supporting the detention. Because the military’s authority to detain individuals stems partly from the AUMF, this allows courts to determine its scope. But habeas rights do not extend to all detainees or to all detention sights, and they can take months or years to be resolved. 

So short of a veto-proof majority in Congress or enormous political pressure, there is little to stop the Trump Administration from further stretching its authority under the 2001 AUMF. While this would surely be the most controversial moment in the AUMF’s long-life, the case is quietly being built as we speak. Whether the Administration decides to make that case, however, is anyone’s guess.