If you had said, one year ago, that the United States would have been militarily involved in a Middle East crisis, few people would have batted an eyelid. President Obama had spent months trying to convince the United States public and Congress that the United States needed to intervene militarily in Syria.

However, the way that this intervention has panned out would surely surprise many. The United States is not intervening against Bashar al-Assad and the Syrian regime, as Obama had proposed. Instead, it is striking one of Assad's biggest enemies, the extremist militia that calls itself the Islamic State. And this intervention is not in Syria, but in neighboring Iraq.

Obama's calls for intervention in Syria ultimately failed, and now bombs are falling in Iraq. Why? There are five big factors.

There was strong opposition to intervention in Syria.

When Obama pushed for military action against the Syrian regime last year, other countries pushed back. Russian President Vladimir Putin, a key ally of Assad's, even went so far as to write a chiding op-ed in the New York Times (which looks a little ridiculous after Crimea, but still). Iran, another key Syrian ally, also pushed back.

Even among European nations that supported the rebels, the support wasn't powerful. In Britain, Prime Minister David Cameron suffered a humiliating political defeat when he asked parliament to support intervention. France, perhaps the most hawkish of the European nations, refused to take the lead.

One key factor was that in both the United States and Europe, multiple polls showed that, even if it was horrified by the Assad regime's excesses, the public just didn't support military intervention.

The Islamic State is both isolated and a threat to the United States.

The Islamic State has few allies. Big, international powers such as Russia do not support it, and important regional powers such as Iran are opposed to it. Even al-Qaeda, the group that birthed the Islamic State, is now against it. While the group is believed to receive funding from groups in Saudi Arabia and Qatar, no one will complain if U.S. forces decimate the Islamic State. Britain and France have also, tentatively, indicated that they may join military action.

Even so, the Islamic State, as isolated as it is, could pose a threat to the United States. If nothing else, it's displaced al-Qaeda as the international face of Islamist extremism: There are now Islamic State "gift shops" in Istanbul, and its flag flies in London. Its practical threat may be limited at present, but in the future it could grow: Norway recently announced it had received a warning that Islamist fighters returning from the Syrian civil war were planning a terror attack.

The Islamic State apparently relishes the idea of taking the fight to the United States. "Don't be cowards and attack us with drones. Instead send your soldiers, the ones we humiliated in Iraq," Abu Mosa, a spokesman for the group, says in a new documentary from Vice News. "We will humiliate them everywhere, God willing, and we will raise the flag of Allah in the White House."

As the Syrian War dragged on, it became less clear who the enemy really was.

The plan to intervene in Syria was based around the idea of routing regime troops. The problem was that as the war dragged on, it became less and less clear who would benefit from such intervention. Would extremist groups such as the Islamic State fill the void? The concern became a sticking point for both right-wing opponents of intervention in the United States and foreign leaders such as Putin, who noted that there were "more than enough Qaeda fighters and extremists of all stripes battling the government."

“Some of the more extremist opposition is very scary from an American perspective, and that presents us with all sorts of problems,” Ari Ratner, a fellow at the Truman National Security Project and former Middle East adviser in the Obama State Department, told the New York Times last April. “We have no illusions about the prospect of engaging with the Assad regime — it must still go — but we are also very reticent to support the more hard-line rebels.” The current situation in Libya, where the post-Moammar Gaddafi political landscape has been dominated by violent extremist groups, seems another warning.

The argument cut the other way, too. If the United States struck the Islamic State in Syria, such action would not only present the Syrian regime with an opening, but it could also set back the Syrian rebels that Washington actually wants to support.

The situation in Iraq right now means that limited airstrikes can make a difference.

So far at least, this is a limited intervention, with limited targets. Laser-guided 500-pound bombs have been dropped, specifically targeting artillery being used by Islamic State fighters to attack Kurdish troops defending Irbil. The current situation in Iraq means that limited involvement like this can have a big impact, and while the Islamic State forces have shown themselves to be smart tactical fighters, they have limited means when fighting an assault from the air.

"Cities aside, most of Iraq looks like a brown billiards table," Andrew Exum, a former U.S. Army officer and prominent expert on the Middle East, tweeted Friday. "Open terrain + artillery/armor pieces = Christmas for USAF/USN aviators."

Iraq's central government and the Kurdish regional powers are also keen for U.S. intervention. As far back as June, the Iraqi government was actually criticizing the United States for not providing air support yet. “This is not only endangering Iraq, but the whole world," Ali al-Musawi, a government spokesman in Baghdad, said at the time.

The United States feels a duty to act.

In a speech defending his decision to take military action in Iraq, Obama mentioned the U.S. military personnel currently in Irbil and how he has a duty to protect them. But it's worth thinking about why those troops are there in the first place. Irbil is a regional capital for Iraq's Kurds, who have proven to be key allies for Washington in Iraq and will be vital in any plans to return Iraq to a functioning state. The United States and Iraq's Kurds have a long, complicated history that also plays a factor here.

Then there's the pressing issue of the Yazidis, a Kurdish-speaking minority that has been targeted by the Islamic State. As many as 40,000 remain stranded on a mountain in Iraq, dying of hunger and thirst, The Post's Loveday Morris reports. "When we face a situation like we do on that mountain," Obama said in a statement late Thursday, "and when we have the unique capabilities to help avert a massacre, I believe the United States of America should not turn a blind eye.” Officials have suggested that the Yazidis' situation could amount to "genocide."

Then, of course, there's the U.S. legacy in the region. Many of Iraq's current problems are a direct or indirect result of the 2003 U.S. invasion of Iraq. While the Obama administration and much of the public at large likely loathe the idea of wider military action, there is still a sense that this is our mess, and we have to help fix it.