A timeline on Syria, from the uprising to Obama’s ‘red line’

The White House is wrestling with how to respond to evidence that Syrian President Bashar Assad’s forces used chemical weapons — which would cross a “red line” set by President Obama.

Obama first warned Assad in August, 2012, that the use of chemical weapons would cross a red line and change his calculus toward Syria.

The White House has taken a cautious approach toward more aggressive action in Syria, saying that it needs to corroborate the evidence of a chemical attack first.

But senior administration officials have said they are now reconsidering providing arms to vetted rebel groups, a decision Obama is expected to make in the coming weeks. Such a move would represent a significant escalation in U.S. involvement in the two-year Syrian civil war.

The following is a timeline showing how the administration has reacted to the threat of a chemical weapons attack and other developments in Syria over the course of the conflict, dating back to the president’s first call for Assad to step down from power 21 months ago:

• Aug. 18, 2011: Obama for the first time calls on Assad to leave power. "We have consistently said that President Assad must lead a democratic transition or get out of the way,” Obama said in a statement as violence in Syria increased. “He has not led. For the sake of the Syrian people, the time has come for President Assad to step aside.”

• Feb. 4, 2012: The U.N. Security Council rejects a U.S.-led UN resolution backing a democratic transition in Syria, due to opposition from Russia and China.

• Feb. 8, 2012: Sens. John McCainJohn Sidney McCainTrump gives nod to vulnerable GOP Sen. McSally with bill signing Democrats need a 'celebrity' candidate — and it's not Biden or Sanders Juan Williams: The high price of working for Trump MORE (R-Ariz.), Lindsey GrahamLindsey Olin GrahamHillicon Valley: House Dems subpoena full Mueller report | DOJ pushes back at 'premature' subpoena | Dems reject offer to view report with fewer redactions | Trump camp runs Facebook ads about Mueller report | Uber gets B for self-driving cars DOJ: Dem subpoena for Mueller report is 'premature and unnecessary' Dems reject Barr's offer to view Mueller report with fewer redactions MORE (R-S.C.) and Joe Lieberman (I-Conn.) say the Obama administration should consider providing arms for the Syrian opposition.

• March 5, 2012: McCain calls for U.S.-led airstrikes in Syria to create safe zones for the opposition. He's the first lawmaker to call for U.S. military action in the Syrian conflict.

• May 27, 2012: Presumptive GOP presidential nominee Mitt Romney calls on Obama to begin providing arms to Syrian rebels, the Republican candidate's first major break with the president on Syria.

• May 31, 2012: As an international peace plan brokered by United Nations envoy Kofi Annan falters, White House press secretary Jay Carney is asked whether there’s a "red line" for the president in Syria. He doesn't address the red line question directly, saying: " I'm not going to preview next steps or options. I can tell you that there is very much an urgent need for action to be taken here, for the international community to further unify and make it clear that a transition has to take place in Syria."

• July 13, 2012: There are reports that the Assad regime is moving its stockpiles of chemical weapons, including sarin gas. Pentagon press secretary George Little says the use of chemical weapons would "cross a serious red line" — which appears to be the first time an administration official says publicly that using chemical weapons is a “red line.”

• July 17, 2012: White House spokesman Josh Earnest says the administration is concerned about the use of chemical weapons by the Assad regime, after Syria's ambassador to Iraq warns upon defecting that Assad will use the weapons if he is cornered.

• July 23, 2012: Obama says that the Assad regime will be "held accountable" if it uses chemical weapons, during a speech at the Veterans of Foreign Wars convention. The same day, a Syrian Foreign Ministry spokesman says that no chemical weapons will be used against Syrians, but he suggests they might be used against "external aggression."

• Aug. 11, 2012: Then-Secretary of State Hillary ClintonHillary Diane Rodham ClintonImpeachment? Not so fast without missing element of criminal intent Former Bush assistant: Mueller report makes Obama look 'just plain bad' Seth Rich's brother calls for those pushing conspiracy to 'take responsibility' MORE says in Istanbul that it’s clear to the Assad regime the use of chemical weapons is “a red line for the world.”

• Aug. 20, 2012: Obama says for the first time that the use or movement of chemical weapons by the Assad regime is a red line. "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," Obama said at a press conference. "That would change my calculus. That would change my equation."

• Sept. 28, 2012: Then-Defense Secretary Leon Panetta says that while Syria's main chemical weapons stockpiles are secure, smaller caches of the weapons are being moved around and U.S. intelligence can't account for all of them.

• Dec. 3, 2012: In a speech at National Defense University, the president again warns Assad over chemical weapons. "If you make the tragic mistake of using these weapons, there where be consequences, and you will be held accountable," Obama says.

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: There are reports that the Pentagon is drafting plans for a preemptive strike against Assad's forces over concerns about the country's chemical weapons stockpiles.

• Feb. 2, 2013: The New York Times reports that Clinton and former CIA Director David Petraeus had crafted a plan in the summer of 2012 to provide vetted rebel groups with lethal arms and training. The White House, in the midst of Obama's re-election bid, rejected the proposal.

• Feb. 7: Panetta and Chairman of the Joint Chiefs Gen. Martin Dempsey tell a congressional panel that they agreed with the plan proposed by Clinton and Petraeus to provide arms to rebel groups.

"What this means is that the president overruled the senior leaders of his own national security team, who were in unanimous agreement that America needs to take greater action to change the military balance of power in Syria," McCain, who prompted the response from Panetta and Dempsey, said after the hearing.

• March 18: Senate Armed Services Chairman Carl LevinCarl Milton LevinListen, learn and lead: Congressional newcomers should leave the extremist tactics at home House Democrats poised to set a dangerous precedent with president’s tax returns The Hill's 12:30 Report — Sponsored by Delta Air Lines — White House to 'temporarily reinstate' Acosta's press pass after judge issues order | Graham to take over Judiciary panel | Hand recount for Florida Senate race MORE (D-Mich.) breaks with the president and says he would go further in Syria, supporting the creation of a safe zone for the opposition. Rep. Eliot Engel (N.Y.) the top Democrat on the House Foreign Affairs Committee, says he is planning legislation to arm vetted Syrian rebel groups.

• March 19: There are reports out of Syria that chemical weapons were used in a town near Aleppo, with more than two-dozen killed. The Assad regime blames the rebels for carrying out the attack, while rebel groups say Assad's forces had used the weapons.

The White House warns there will be "consequences" if a chemical attack took place, but says it can't confirm the use of chemical weapons and is "looking carefully at this information as it comes in." Administration officials say they don’t believe rebel forces used any chemical weapons, however.

• March 20: Obama says at a press conference that the use of chemical weapons would be a "game changer" and adds that his administration is going to investigate the reports of a chemical attack.

• April 11: Senate Foreign Relations Chairman Robert MenendezRobert (Bob) MenendezWe can accelerate a cure for Alzheimer's The Hill's 12:30 Report: Manafort sentenced to total of 7.5 years in prison Acting Defense chief calls Graham an 'ally' after tense exchange MORE (D-N.J.) joins the list of lawmakers who support providing arms to vetted Syrian rebels.

• April 25: In a letter to Congress, the White House says that the intelligence community assesses “with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin.”

The letter prompts a slate of lawmakers from both parties to call for more U.S. action in Syria, including creating a no-fly zone and providing arms to rebel groups.

• April 26: Obama expresses caution toward taking more aggressive action in Syria in his first comments on U.S. assessment that the Assad regime used chemical weapons. He reiterates that it would be a “game changer” but says that more evidence is needed to verify that Assad’s was behind the chemical attacks.

• April 30: At a news conference, Obama again strikes a cautious tone, saying he wants to get the facts and corroborated evidence before making any decisions. Asked what he would do if the chemical attacks were verified, Obama says “a range of options” are available.

“Clearly that would be an escalation, in our view, of the threat to the security of the international community, our allies and the United States, and that means that there are some options that we might not otherwise exercise that we would strongly consider,” he says.

Only hours later, The Washington Post reports that the president is moving closer toward choosing to arm vetted rebel groups.

• May 2: Defense Secretary Chuck HagelCharles (Chuck) Timothy HagelSwalwell says he will convene a bipartisan 'blended cabinet' if elected president Overnight Energy: John Kerry hits Trump over climate change at hearing | Defends Ocasio-Cortez from GOP attacks | Dems grill EPA chief over auto emissions rollback plan For planet and country: National security's climate moment MORE confirms that that the administration is reconsidering its opposition to providing arms for the opposition. Obama says at a news conference in Mexico later that day that Hagel’s comments were “what I've been saying now for months, which is we are continually evaluating the situation on the ground.” Obama again cautions that the United States must “look before we leap” in Syria.