President Donald Trump ordered military strikes against Syrian President Bashar Assad’s regime on Thursday, a decision that follows years of U.S. leaders weighing how to respond to a conflict that has killed hundreds of thousands of people and displaced many more.
Here is a look at how the United States has dealt with the violence in Syria since the 2011 start of the civil war, one of several uprisings in a series of Middle East protests known as the Arab Spring.
Spring 2011: Protests erupt around Syria
The uprising that would later lead to civil war started with pro-democracy demonstrations in protest of Assad’s regime in early 2011. Around March, the protests gained steam, and demonstrators increasingly called for the overthrow of Assad.
August 2011: Obama calls for Assad to resign
Then-President Barack Obama formally called on Assad to step down “for the sake of the Syrian people” and issued new sanctions on his regime in August 2011. Several allies — President Nicolas Sarkozy of France, Chancellor Angela Merkel of Germany and Prime Minister David Cameron of the United Kingdom — called on Assad to resign too.
August 2012: Obama calls chemical weapons a “red line” in Syria
In remarks to reporters at the White House, Obama warned Assad’s regime not to use chemical weapons, describing it as a “red line” for his administration that could prompt the United States to intervene militarily.
“We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized. That would change my calculus,” Obama said.
August 2013: Chemical weapons attack kills hundreds in Damascus
Chemical weapons were used in the conflict at points earlier in the year, but pressure built on Obama to act on his “red line” when Syria used them to kill civilians in a brutal August 21, 2013, attack on rebels in Damascus. Nearly 1,500 civilians, many of them children, were killed.
As detailed in an American intelligence assessment, the U.S. charged that the Syrian government was behind the attack and said high-ranking officials in Assad’s regime were involved.
August 2013: Obama asks Congress to approve strikes; proposal lacks support
In response to the chemical weapons attack, the Obama administration signaled that the president was considering “limited” military intervention in Syria. Obama said he wanted Congress to authorize the attack, saying he was “mindful that I’m the president of the world’s oldest constitutional democracy.”
Not yet working in government, Donald Trump took to Twitter to urge Obama to warn against involving the United States in the conflict.
The proposal to take military action was met with skepticism from some members of both parties in Congress, and especially the House, which was controlled by Republicans.
September 2013: U.S. reaches deal with Russia to curtail Syria’s chemical weapons
In early September, the Senate Foreign Relations Committee advanced a proposal granting Obama the authority to take some military action against Syria. But momentum for a full Senate vote on such a resolution soon faded, and Congress never authorized Obama to use force.
Instead, the United States and Russia, an ally of Assad’s regime, hashed out a deal setting a mid-2014 deadline for removing or destroying Syria’s chemical weapons.
June 2014: Last of “declared” chemical weapons said to leave Syria
The watchdog Organization for the Prohibition of Chemical Weapons signaled that the last of Syria’s “declared” chemical weapons arsenal had been successfully removed from the country in June, as called for by the U.N. Security Council the previous year.
But the organization’s leader, Ahmet Uzumcu, cautioned that “we cannot say for sure it has no more chemical weapons.”
September 2014: Congress approves plan to arm Syrian rebels; Obama orders air strikes
Later in the year, Obama asked lawmakers to approve his plan for the United States to arm and train rebels in Syria to fight the Islamic State in the region. Congress approved it with bipartisan support.
The United States also carried out air-strikes against ISIS in Syria, with help from an international coalition that included the United Arab Emirates, Saudi Arabia, Qatar, Bahrain and Jordan.
August 2016: Syrian government carried out gas attacks, investigation finds
An investigation led by the U.N. and Organization for the Prohibition of Chemical Weapons found that the Syrian government used chemical weapons, namely the banned toxic chlorine gas, in attacks in 2014 and 2015.
ISIS had used mustard gas, the inquiry also concluded.
January 2017: Trump orders ban on Syrian refugees
Trump, now president, signed a controversial executive order establishing an indefinite ban on accepting refugees from the country. The ban, which the Trump administration asserted was necessary for the nation’s security, was quickly halted by the courts after critics said it discriminated against Muslims.
A second version of the travel ban, which is working its way through the courts, would not ban Syrian refugees indefinitely.
April 2017: Chemical attack, attributed to Assad’s regime, kills dozens in Syria
Dozens of people, including children, died in a chemical attack in Syria that the United States and others say was carried out by Assad’s regime. Officials in Turkey said initial tests on some victims suggested that sarin, a banned nerve gas, was used in the attack.
Trump and other American officials condemned the attack as horrifying, inhumane and a breach of the 2013 treaty. Trump said seeing footage of the victims on television had affected him, and he described the attack as crossing “many lines” for him, “beyond a red line.”
April 6, 2017: Trump orders strikes against Syria in response to chemical attack
The White House announced that the U.S. was taking military action against Assad’s regime in the form of airstrikes.
Some lawmakers, including some Republicans, called on Trump to seek congressional approval for the use of force. Sen. Majority Leader Mitch McConnell, however, defended the president’s action as necessary and said he does not need to seek authorization from Congress.