Since the Syrian uprising began in spring 2011, President Obama has been reluctant to get involved in the violent clashes between the government led by Bashar Assad and the rebel opposition.

Despite this caution, Obama shocked his own national security team by warning Assad in August 2012 that the use of chemical weapons would cross a “red line” and change his calculus toward Syria.

In the months following, even in the face of small-scale chemical weapons attacks, Obama dragged his feet in ordering a major U.S. response by saying that the intelligence community needed more evidence that chemicals were deployed or by putting off military action by agreeing to provide more support to the rebels.

But reports of a large-scale chemical attack by Assad’s forces in eastern Damascus last week left Obama little choice but to launch a military strike or face credibility concerns at home and abroad.

Here’s a timeline of how we got here:

March 15, 2011: Uprising against the Syrian government begins.

April 8, 2011: Obama calls on Assad to halt the “abhorrent
 violence committed against peaceful protesters.”

April 22, 2011: Obama condemns the Assad regime’s use of force against
demonstrators and calls on Assad to “change course now.” “This outrageous use of violence to quell protests must come to an end now,” he said.

April 29, 2011 – Obama imposes new sanctions on Syria and calls on U.S. allies to follow suit.

Aug. 11, 2011 – Obama and Turkish Prime Minister Tayyip Erdogan agree to set a deadline of Aug. 27 for Assad to stop the uprising and transition to genuine democratic reforms in the country, including free elections. If Assad fails to make good on promises to implement democratic reforms, Obama and Erdogan pledge to talk again about a potential U.S.-Turkish military operation in Syria.

Aug. 18, 2011 – Obama and U.S. allies call for Assad to step down after his regime flouts agreements to implement democratic reforms and steps up his slaughter of protesters.

Oct. 24, 2011 – U.S. withdraws its ambassador to Syria, Robert Ford, over fears for his safety. Ford had angered Syrian regime over his support for anti-Assad protesters.

May 18, 2012 – Group of Eight leaders at Camp David 
broadly agree on “the need to move rapidly toward a plan for for political transition within Syria.” Russian President Medvedev does not outright support the call but doesn’t oppose it, either.

June 18, 2012 – Obama and Russian President Vladimir Putin 
at the Group of 20 summit in Mexico meet and agree that violence in Syria must end. They pledge to work with the international community, including the United Nations’ Kofi Annan, in trying to find a resolution to the civil war.

June 22, 2012 – Defense Secretary Leon Panetta defends 
the administration’s decision not to arm the Syrian rebels,
which Republicans such as Sen. John McCain, R-Ariz., and others had criticized. Panetta said he “thinks it’s very important right now that everybody focus on a smooth and responsible political transition” in Syria so it doesn’t “deteriorate into a terrible civil war.”

July 18, 2012: Obama phones Putin to discuss “out of control” Syria and the escalating violence after a bombing in Damascus
 kills members of Assad’s inner circle, and Panetta warns that the country was “rapidly spinning out of control.” Obama and Putin agree only on the need for a political transition and an end to the violence.

July 19, 2012: China and Russia members of the U.N. Security Council veto a U.N. Security
 Council resolution that would have pressured Assad to end the violence against the uprising. A Russian official said the measure would have given Western countries leverage in overturning the government. Then-U.N. Ambassador Susan Rice accused Russia and China of protecting Assad, who she described as “dangerous and deplorable.”

Aug. 20, 2012: In a rare press conference, Obama warns Assad for the first time that using chemical weapons would amount to a red line violation of international norms. “A red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized,” he said. “That would change my calculus. That would change my equation.”

Dec. 3, 2012: In a speech at the National Defense University in Washington, Obama again warns Assad that the use of chemical weapons would be “totally unacceptable.” “If you make the tragic mistake of using these weapons, there will be consequences and you will be held accountable,” he said.

Then-Secretary of State Hillary Clinton tells reporters in Prague that the U.S. is “planning to take action” if Assad’s regime launches a chemical attack.

Dec. 7, 2012: Amid reports that the Syrian regime has ordered forces to load precursor chemicals for the nerve gas sarin into bombs, the Pentagon drafts plans for a preemptive strike on Syria.

Feb. 7, 2013: Internal administration divisions over arming the Syrian rebels are exposed in a congressional hearing. Panetta and Joint Chiefs Chairman Gen. Martin Dempsey tell McCain they supported the plan from the previous summer to arm carefully vetted Syrian rebels. The admission shows that Obama rejected the advice despite near unanimity from his national security team on arming the rebels.

March 18, 2013: Sen. Carl Levin, Michigan Democrat and chairman of the Senate Armed Services Committee, breaks with Obama. Levin says he would go “further than the president” and suggests establishing a “safe zone” along the Turkey-Syria border for refugees looking to escape the violence.

Rep. Eliot Engel, D-N.Y., the top Democrat on the House Foreign Affairs panel, says he would introduce a bill providing up to $150 million to arm the Syrian rebels.

March 19, 2013: The Assad regime and rebels accuse each other of launching a deadly chemical attack in the city of Aleppo, the first alleged use of such weapons in the conflict. More than two dozen are killed.

The White House reiterates its threat that there will be consequences if a chemical attack took place, but says it will evaluate evidence as it comes in.

March 20, 2013: Obama says the use of chemical weapons in Syria would be a “game-changer” that would demand international action. He says the U.S. and other nations are trying to determine whether those weapons were used.

April 11, 2013: White House says the intelligence community “with varying degrees of confidence” has assessed that the Assad regime used chemical weapons on a small scale in Syria, specifically mentioning sarin. Administration increases aid to Syrian rebels.

April 30, 2013: Obama strikes a cautious note on claims that Syria used chemical weapons. He says the U.S. and global partners are investigating and says the probe would seek “more direct evidence and confirmation.” Even so, he says there are a “range of options” at his disposal if the attack is confirmed.

April 26, 2013: Defense Secretary Chuck Hagel says the U.S. is rethinking its opposition to arming the Syrian rebels, but Obama says no decision to deepen U.S. involvement is imminent.

May 7, 2013: Obama defends his reluctance to increase intervention in Syria, saying he has yet to receive corroborating evidence to back up initial reports of small-scale use of chemical weapons by Assad and would not make a decision based on a “hope and a prayer.”

May 13, 2013: British Prime Minister David Cameron takes a more aggressive stance on Syria’s civil war than Obama during a press conference, signaling heightened international concerns about the Syrian opposition’s fate.

June 14, 2013: White House officials say Obama has decided to provide “military support” after acknowledging for the first time that Assad used chemical weapons against rebels.

June 21, 2013: Syrian rebels say weapons from the U.S. have not materialized but the U.S. decision to offer assistance prompted other countries to send shipments of heavy weaponry.

July 23, 2013: Dempsey, in a letter to Congress, says establishing a no-fly zone over Syria would cost the U.S. $500 million to $1 billion and would not quell the conflict. The no-fly zone is just one option, Dempsey says. All the others would cost $1 billion a month.

Aug. 20, 2013: Reports surface of a large-scale chemical attack in Ghouta, a village in Eastern Damascus. Stream of videos emerge showing rows of dead children and patients convulsing in their beds.