When President Obama announced Thursday that he had authorized air strikes against ISIS, the extremist Sunni group that has seized large parts of Iraq and Syria, he did not use the phrase "red line," but the implication was clear.

The United States will use military force against ISIS if — and only if — the group threatens Iraq's largely autonomous Kurdish region. That is the red line.

"To stop the advance on [the Kurdish capital city of] Erbil, I've directed our military to take targeted strikes against ISIL terrorist convoys should they move toward the city," Obama said. He also said he had "authorized targeted airstrikes, if necessary, to help forces in Iraq as they fight to break the siege of Mount Sinjar [where thousands of civilians have fled from ISIS] and protect the civilians trapped there."

Obama is drawing a red line around Iraqi Kurdistan -everyone outside of it is on their own

If you are a member of ISIS, here is how you might hear Obama's message: Stay away from Iraqi Kurdistan, and the rest of northern Iraq is yours to keep. Based on Obama's words and actions so far, you would not be so wrong.

ISIS first swept through northern Iraq in June, invading from the territory it had already conquered in Syria. The group took Iraq's second-largest city, Mosul, as well as much of Iraq's largely Sunni north. The US sent military advisors to Iraq and evacuated its diplomats from Baghdad to the much-safer Kurdish region, but it did not take any military action against ISIS. The group had stopped short of Baghdad and had not pushed into Iraq's Kurdish region.

America's calculus changed over the last few days, as ISIS has made its first real push into the Kurdish region, taking some Kurdish towns, encroaching awfully close to the Kurdish capital of Erbil, and creating a humanitarian catastrophe by forcing ten thousand or more Yazidi minority civilians out of their homes and onto a nearby mountain, where they are waiting to die of starvation or ISIS's guns.

Invading Iraq's Kurdish region, it turned out, was Obama's red line for ISIS. There are a few reasons why. The Kurdish region is far stabler, politically, than the rest of Iraq. (Kurds are ethnically distinct from the rest of Iraq, which is largely ethnic Arab; most Kurds are Sunni Muslims.) The Kurdish region, which has been semi-autonomous since the United States invaded in 2003 and has grown more autonomous from Baghdad ever since, also happens to be a much more reliable US ally than is the central Iraqi government. It has a reasonably competent government and military, unlike the central Iraqi government, which is volatile, unstable, deeply corrupt, and increasingly authoritarian.

It's not hard to see how a cost-benefit calculation might lead the Obama administration to choose defending just Iraqi Kurdistan over defending all of Iraq from ISIS: the Kurdish region is smaller, it already has a competent military on the ground, it is reliably pro-US, and it can probably be protected at much lower risk to the US. With the rest of Iraq in chaos, the Kurdish region is also America's last reliable base in the country, so if Erbil falls to ISIS then the US could effectively be out altogether.

Defending all of Iraq against ISIS, meanwhile, would be much more difficult. Iraq's military is corrupt and has previously deserted in large number against the insurgents, meaning the onus would be on US or other foreign troops; ISIS already controls huge parts of the country, so booting them out would require a full invasion force. Re-taking all of northern Iraq would be very costly and high-risk for the US, and while the reward would be humanitarian relief for many northern Iraqis, it could also force the US to once again military occupy a part of the world that Obama worked very hard to remove US troops from.

kicking isis out of northern iraq could require another us occupation

That is likely how Iraqi Kurdistan became Obama's red line for ISIS. On a background briefing call with White House officials late on Thursday, the emphasis on defending Erbil came through loud and clear: the US is clearly designing its intervention around protecting the Kurdish region; any effect for the rest of Iraq is secondary, and was premised on Iraq's government first fulfilling some political commitments.

The effect, though, is to imply that the US will not intervene against ISIS if they remain on the correct side of the red line — effectively giving them the US go-ahead to continue terrorizing the vast territory in northern Iraq they've already seized.

This is the problem that Obama created with his 2012 red line for Syria, in which he stated that the US would intervene against the Syrian government if it used chemical weapons against civilians. The implication was that the US would not intervene if Syria continued using conventional weapons to slaughter many thousands of civilians, which Syria did with great enthusiasm. Syria did end up crossing that red line, frequently and egregiously, and the US did not really punish it very much, although that may have more to do with deeper differences between Syria and Iraq.

So there is good reason to worry, based on Obama's record with his Syria red line, that he might not fully follow through on defending Iraqi Kurdistan from ISIS as he seemed to promise on Thursday. Still, there are some key distinctions: the US has broad international support, at least so far, for Iraq in a way that it did not for Syria. The US has the support of the Iraqi government, whereas in Syria the government was its opponent. And it is much easier to measure whether the red line is crossed: Syria wore down its red line by using very small sarin gas attacks, and in a way that left ambiguity as to what had happened, so that by the time it was gassing whole neighborhoods the effect had been gradual and thus easier for the world to ignore. That's less true in Iraq; either ISIS invades Erbil or it doesn't, so the red line is clearer and more categorical.

It is possible that Obama's calculus on this could change, or that he is quietly planning to support a larger Iraqi push to kick ISIS out of the rest of the country. But, on the surface, for now, it certainly looks like America is drawing a giant red line around Iraqi Kurdistan – and telling everyone outside of it that they're on their own.