The Syrian war has been a slowly unfolding catastrophe, one that has embroiled several of the world's major powers. Here is a look at how the war started, why it became so complicated and what might happen next.

How did the war in Syria start?

In March 2011, peaceful protests broke out in the country as part of the Arab Spring uprisings. Organizers called on Assad to undertake democratic reforms, but his government responded with violence. Some of the protesters then teamed up with military defectors to form the Free Syrian Army, which sought to overthrow the government. By 2012, this armed struggle had devolved into a full-blown civil war.

What is Russia doing in Syria?

Russia has long supported Assad’s government. According to one scholar, Russia helped build the modern Syrian military, and Assad is one of Russian President Vladimir Putin's strongest allies in the Middle East. Russia has often blocked meaningful international action on Syria by vetoing proposals in the U.N. Security Council, and Moscow changed the course of the war in Assad's favor by beginning a military intervention in 2015.

Russia has stood by Assad's side in other ways, too. The Russian government has backed up Assad's claims that he did not use chemical weapons in the recent attack. Russian officials have gone so far as to suggest the event was “staged.” In response, Haley accused the Russians of enabling Assad. She promised further sanctions on Russia.

Why is the United States involved?

The United States has been reluctant to become too entangled in Syria, but it has acted for two main reasons.

First, the Islamic State militant group began gaining a foothold in the country in 2013. The next year, the United States launched airstrikes against the group. It eventually sent ground troops into the fight, and about 2,000 U.S. forces are now deployed there.

Second, the United States has acted to punish the Assad government for using chemical weapons such as sarin and chlorine gas on civilians.

How has the United States punished Assad?

In 2012, President Barack Obama called the use of chemical weapons a “red line” that would prompt military intervention. The following year, a sarin nerve agent attack in the Eastern Ghouta region near Damascus killed about 1,400 people, according to the U.S. government's assessment. Obama pushed for a strike but could not get congressional approval.

Instead, the international community pushed a diplomatic solution. The U.N. Security Council ordered Assad to destroy his chemical weapons stockpile and sign the Chemical Weapons Convention that prohibits countries from producing, stockpiling or using chemical weapons.

There have been several reports of chemical weapons attacks since then. One particularly horrific strike occurred April 4, 2017. Nearly 100 people were killed in the northern town of Khan Sheikhoun in an apparent sarin attack. It garnered worldwide headlines and the attention of Trump, who expressed horror at the images of “innocent children, innocent babies” poisoned by a nerve agent. Just a few days after that attack, he authorized a cruise-missile strike on a Syrian air base. It was the first direct U.S. strike on the Syrian regime in the war.

How long will the United States remain involved in Syria?

Not for much longer. In December 2018, Trump administration officials said all U.S. troops will withdraw from Syria immediately. Trump said the troops had accomplished their mission in Syria by defeating the Islamic State. But many experts say the militant group is far from beaten. In fact, they predict it will only grow stronger and more violent in the months ahead.

Who are the key regional players helping Assad?

One of Assad's key allies is Iran. Iran needs Syria to move its weapons and proxy militias across the Middle East. So when Assad seemed threatened, Iran stepped in to support him. So did Hezbollah, the Lebanese political party and militia that is a close ally of Tehran.

That upset Iran’s rivals in the region, including Saudi Arabia and Turkey. So they began sending arms and money to anti-Assad rebels, including extremist militias. Meanwhile, Israel has been carrying out strikes on Syrian military bases and other facilities on its own.

That means that today, many wars are being fought inside Syria. The Syrian government is still at war with the rebels. Israel is at war with Iranian-backed forces. And the United States is trying to kill off the Islamic State.

Oh, and there is another conflict, too. Syria is home to a significant number of Kurds, a key U.S. ally. Turkey fears that the Kurds aim to establish a separate state and has been bombing Syrian cities under Kurdish control.

What has life been like for Syrians?

Horrible. Syrian families often do not have basic necessities, such as food, shelter and medical care. Children cannot go to school. The war is being fought in towns and on the streets.

Since the start of the war, more than 465,000 Syrians have been killed, 1 million have been injured and 12 million — more than half the country's population — have been forced to flee their homes. Of the 12 million displaced, more than 5.5 million have moved abroad and registered as refugees.