One way or the other, the United States is about to take responsibility for what happens in Syria’s gruesome civil war.

President Obama stopped the rush to military action by calling for Congress to preapprove an attack to punish the regime for using chemical weapons. But if Congress votes yes, the United States will be a party to the conflict, no matter how limited its actions. If Congress votes no, Washington will have walked away from what Obama called “a crime against humanity” and will be blamed for the carnage to come. It’s not a pretty picture, and it’s extremely complicated.

Here are five key points to bring you up to speed on Syria:

(1) This is a full-blown civil war. The ruthless Assad family has ruled Syria since 1970. It is essentially a mafia operating through its Baath Party organization and the military. The Assads come from the minority Alawite sect of Islam, and so do most of the key commanders in the Syrian armed forces. They see this as a fight for survival, and no sign of weakness is allowed. Bashar al-Assad, who inherited the presidency from his father in 2000, could not make any deal that cut out his family and the clans that support it, even if he wanted to.

On the other side of the ill-defined battle lines are militias drawn largely from the country’s Sunni Muslim majority: they include ex-Syrian military officers, farmers, and professionals. But among the most effective fighters are those affiliated with the Nusra Front, acomponent of al Qaeda. They are intimately allied with jihadists in Iraq who fought against the United States for most of the last decade. They have declared this a jihad against the Assad regime and called on would-be holy warriors from around the world to join their cause. Many have. One nightmare scenario in this war is that the Assads fall and al Qaeda gets hold of their chemical arsenal.

(2) This is also a “proxy war.” The factions are backed by different countries with different objectives that are trying to settle their scores on Syrian land with Syrian lives.

The alliance between the Assad regime and Iran goes back 30 years. The powerfulHezbollah militia in neighboring Lebanon, originally created by Iran and Syria, is another key ally of the Assads’. So is Russia, which has backed them since the Cold War and is betting that it can restore some of its lost influence in the region and the world if they hold on to power.

On the opposition side, the Saudis see Iran as their greatest strategic threat and Hezbollah as a terrorist problem. Bring down the Assads, and you cripple the mullahs in both Iran and Lebanon. The Saudis also want to undermine the Muslim Brotherhood and its operatives, even though the Brethren play a large political role in the Sunni opposition. Qatar gives the Brethren money, and Turkey gives them strategic depth by letting them operate out of its territory. There is a risk that Saudi Arabia will back the more radical factions in the hope it can hurt Iran and weaken the Brotherhood at the same time.

The U.S. administration says, vaguely, that the opposition will be getting more support. The United States reportedly has trained rebel factions deploying out of Jordan, but there is no indication they have the numbers or the prowess to influence the outcome of the war at this point. Israel, meanwhile, does not have any major faction to support in the war. As long as the Syrian conflict does not cross its border, it is content to let its old enemy bleed. But Israel has to worry whether, if Obama draws a red line against chemical weapons in Syria and doesn't enforce it, he can be trusted when he says he has drawn a red line against nuclear weapons in Iran.

(3) The Obama administration wants a negotiated end to the fighting. That’s not surprising, given the unsavory characters on both sides. But that is not going to happen until the combatants are much more exhausted than they are now. “It’s like a forest fire,” says Ryan Crocker, a veteran U.S. diplomat who served as ambassador in Lebanon, Syria, Pakistan, Iraq, and Afghanistan. “This one is burning way too hot for any effort to extinguish it to succeed. Maybe when both sides have worn themselves out, there will be a role for diplomacy.” The Obama administration’s objective, horrible as it sounds, is basically to help maintain a stalemate. Unfortunately, as we saw during the 15-year civil war in Lebanon, long after the people want to give up fighting, outside forces keep pushing them to kill and to die. The Bosnian war in the mid-1990s is another example. More than 200,000 died there before a peace accord could be hammered out.

(4) Chemical weapons are a game changer. Consider this: more than 100,000 Syrians have died in two years of fighting, and there are now 2 million Syrian refugees who've fled their country. Millions more have been displaced internally. So why claim that when 1,400 people are killed by chemical weapons, this is the red line that cannot be crossed? Obama and Secretary of State John Kerry emphasize the proclamations of the international community, dating back to 1925, that declare chemical weapons should be forever banned from the world’s battlefields. But there is a much more practical and immediate concern. Chemical weapons, especially those like sarin that are colorless, odorless, and highly lethal, are tools of terror. Already, the commanders of some opposition forces say they are reluctant to press their offensives lest they, or the civilians around them, be gassed again.

Here the example of Iraq is highly instructive. In 1988 Saddam Hussein used gas to slaughter thousands of men, women, and children in the Kurdish city of Halabja. Three years later, at the end of the Gulf War, when he had been defeated in Kuwait, but his helicopters were still flying over Iraq, hundreds of thousands of panicked Kurds fled their homes and froze on mountainsides near the Turkish border. He had not used gas against them again. He didn’t have to. They knew its effects, and they were simply terrified.

In an increasingly sectarian war, Assad could use gas as the ultimate tool for ethnic cleansing—unless he is convinced that doing so will endanger him and his regime.

(5) If Obama orders a strike, he wants to pick the place and time. The next few days and weeks will see fierce politicking and diplomacy as the Obama administration tries to push a resolution through Congress that supports its action. There will also be intense diplomacy with the Arab League (backed up by Saudi clout), during the G20 meeting that convenes Thursday in St. Petersburg and at the U.N. General Assembly, which begins September 17. In the meantime, the Assads will try to hide their most important weapons. But if later this month Obama carries out an act of war as planned, he will be able to marshal a great many more political and diplomatic resources than he would have over Labor Day weekend.