The American war against ISIS is on shaky legal ground. The Obama administration doesn't have express authorization from Congress to bomb ISIS in Iraq and Syria, so it's claiming — dubiously — to derive legal authority from a 2001 law called the Authorization for the Use of Military Force (AUMF), which was passed to approve war against al-Qaeda after 9/11, but became the authorization for a much wider global war on terror.

Now, the Obama administration is asking Congress to pass a new AUMF, one written to authorize its war against ISIS. But there's much more at stake here than just giving Obama permission to keep bombing ISIS. In writing a new AUMF, Congress will either outright replace the old 2001 law, or even if it lets the 2001 law stand it will have to consider how the new and old laws interact, potentially modifying the sweeping presidential powers in fighting terrorism.

Either way, authorizing Obama's war on ISIS will mean potentially tinkering with the 2001 AUMF, which matters for much more than just ISIS. The 2001 law became the central legal basis for the entire global war on terror, from invading Afghanistan to drone strikes to secret renditions.

The debate over an ISIS AUMF, then, isn't just a debate about ISIS. It's a debate over whether Congress should see this as an opportunity to roll back the seemingly endless, limitless war on terrorism, keep that war in place, or extend it beyond even what it was under George W. Bush. Here are the three ways this could play out, who's pushing for them, how they'd work, and what they would mean for American power.

Congress could roll back presidential powers in the AUMF

The issues with the AUMF "basically amount to who, what, where, [and] when," according to Ben Wittes, a senior fellow in Governance Studies at The Brookings Institution. That means what groups the president is allowed to target, what tools he can use to go after them, where he's allowed to attack them, and how long this war can go on for.

the new bill could stamp an expiration date on the war on terror

Theoretically, an ISIS AUMF could reform any and all of these. A new bill could limit the war — currently all but limitless, under the 2001 law — to only ISIS, al-Qaeda, and maybe the Taliban. It could geographically limit the war to Syria, Iraq, Afghanistan, and maybe a handful of other countries. It could also ban the use of ground troops, or prohibit Guantanamo-style detention in the future. "The courts have interpreted the AUMF to include the power to detain the enemy, hence Guantanamo," Wittes says. An ISIS AUMF could change that.

Perhaps most importantly, the new bill could stamp an expiration date on America's global war on terror. "One of the main criticisms of the AUMF is that it authorizes endless war," Wittes says. "That's a policy choice. You could have a sunset provision that forces Congress to come back in a year or two years and re-up."

Such reforms in themselves wouldn't fully end the war on terror. Wittes and several colleagues wrote a draft AUMF that contains a number of these changes — and he thinks it's not impossible that a bill like theirs becomes law.

"The person closest to the spirit of that paper in the Senate is Tim Kaine," Wittes says. Kaine, a Democrat from Virginia, "wants to provide an authorization that is also a set of limitations because he doesn't want to be in the business of authorizing endless war."

Before the midterm elections, Kaine had written a draft authorization that would put fairly tight limits on the ISIS war. Another draft, by Rep. Adam Schiff (D-CA), would fully sunset the 2001 AUMF after 18 months — in other words, it would force Congress to reauthorize the AUMF if it wanted the president to retain the sweeping powers the law granted.

A lot of this turns on which party drives the legislation for the new law to authorize war on ISIS. If Democrats end up writing the bill, there's a real chance some of these reforms could become law.

Before the elections, there was "largely a Senate-led [reform] effort that the White House would have been totally supportive of," according to Ken Sofer, the the associate director for national security and international policy at the Center for American Progress (where I worked for a little under two years, though not with Sofer directly).

Sofer says that these Democrats, with some Republican support, wanted to "get rid of the 2001 AUMF, get rid of the 2002 AUMF [the law permitting the 2003 Iraq war], and make something that is specifically focused on ISIS and closes down expansive interpretations for being affiliated with or fighting with al-Qaeda." This kind of bill is more likely if an ISIS AUMF passes during the lame duck session, before Republicans take control of the Senate. As long as Democrats hold the majority, they're in a better position to have a significant role in shaping the bill.

House Speaker John Boehner has said he doesn't want to vote on an AUMF until after the new Senate takes its seats. We'll just have to wait and see how negotiations between the White House, Senate, and House shake out between now and January. But it looks more likely that Republicans will hold both houses of Congress when it comes time to draft a new AUMF.

Congress could expand the powers in the AUMF

"If the Senate GOP is writing the AUMF," Sofer says, it could well be "expanded in a way that's going to give even more unilateral authority to the executive."

There are lots of ways this could happen. One of the few limiting principles in the 2001 AUMF is the idea that any targeted groups have to have been somehow involved in the 9/11 attacks or involved with groups that have. In practice, this hasn't placed huge constraints on the president, but an ISIS AUMF could remove even that limitation. It could authorize Obama to target any militant group anywhere that he believes poses a threat to the United States, enshrining a limitless forever war into law.

Moreover, Congress could expressly authorize the use of ground troops against ISIS and other groups in Iraq and Syria — in effect instructing the White House to invade. That decision is ultimately up to the president, not Congress, but it would be, if nothing else, a way to very publicly pressure Obama to escalate.

It could authorize Obama to target any militant group anywhere

This sort of bill is pretty possible. Many of the leading Republican senators on this issue tend toward hawkishness — Lindsey Graham and John McCain, for example, or incoming Senator Tom Cotton.

According to Wittes, these Republicans "want a broad authorization that gives the President not only the power but the instruction to go on offense" against ISIS and similar groups.

Some Republicans are already explicitly discussing using this moment to expand the old AUMF to expand presidential powers. In September, House Armed Forces Committee Chairman Buck McKeon wrote a letter to the administration calling for an "augmentation of the 2001 AUMF" or a new AUMF that allows the president to use "all elements of national power" (emphasis McKeon's).

These Republicans can count on support from the bulk of their part, as well as some more hawkish Democrats. Republicans tend to strongly support taking a more aggressive policy toward ISIS.

If the Senate GOP rather than Senate Democrats end up writing the AUMF, the hawkish voices that dominate the GOP leadership will necessarily have more influence on the drafting process. That means a bill is more likely to be written to their specifications — potentially expanding the war on terror beyond even the 2001 framework.

Congress could maintain — and deepen — the status quo

But there's a third option: Congress does nothing, or does act but in a way that maintains the status quo of the 2001 law. "This is gonna be a non-story and a non-event," James Jay Carafano, Vice President for Foreign and Defense Policy Studies at the Heritage Foundation, predicts.

One way to do this would be to simply pass no new authorization at all, though both Carafano and Wittes believe that's unlikely. Democrats will support the administration's push for a new law, they argue, and Republicans won't miss an opportunity to vote in support of the war against ISIS.

Carafano, at the conservative Heritage Foundation, believes that the more likely scenario is that Congress will pass a bill that permits a war with ISIS, but one that doesn't really affect the larger war on terror or the 2001 law authorizing it. "There may be some restrictions in there," Carafano says, "but I don't think they'll be very meaningful."

Even if there's disagreement about broader war on terror issues, he argues, there's basic bipartisan agreement that the US should be in the business of fighting ISIS. Therefore, they won't tie the ISIS issue to more controversial issues such as Guantanamo or drone strikes. "There may be language that seems prohibitional, like geographic limitations," he says, "but they'll probably have wiggle room to get out of that.

Until the new new senators take their seats, we have no idea how the Senate's flip from Democratic to Republican control will affect the way Congress approaches this issue. Nor do we know whether a lame duck bill could go through. It's just too soon to judge.

In any outcome, the future of one of America's most important foreign policy issues — the seemingly endless war on terror — will necessarily come up for debate. If Congress decides to expand or roll back that war, or even if the outcome is that Congress chooses to leave the old rules in place, it will be a rare and potentially pivotal moment of reflection on the decade-plus-long global war on terror.