WASHINGTON – Last Thursday  marked the seven-year anniversary of  the beginning of the Syrian civil war, a shameful period that has led to the deaths of as many as 500,000 people.

Syrian President Bashar al-Assad once appeared on the verge of defeat at the hands of rebels before Russia and Iran intervened on his side. Now, as his regime is crushing the last pockets of resistance in the eastern suburbs of Damascus, one major question looms over the conflict:

What prevented former-President Barack Obama from stepping into the conflict while Assad was losing his grip and likely toppling the brutal regime?

The now-infamous  2013 “redline” incident is the moment the Obama administration could have stepped into the Syrian war and altered history – but chose to step back.

The previous year,  Obama told reporters during a press conference that Assad would suffer  “enormous consequences” if his  regime used chemical weapons against his own people. He threatened airstrikes to enforce his chemical weapons “red line” in Syria.

However, when Assad executed another chemical weapons attack in the suburbs of Damascus that killed over 1,400 people, Obama backed away from a military response and pushed the decision to a dithering Congress.

The administration would later strike a deal with Russian President Vladimir Putin to oversee the removal of chemical weapons by Assad’s government and draft a United Nations resolution intended to prevent the future use of chemical weapons in Syria.

Obama praised the deal, saying that he found a diplomatic solution for the U.S.—a country that had spent  the last decade entangled in bitter wars in the Middle East. But the administration was continually criticized for its decision – which seemed spineless and built on a tissue of improbable assumptions about the future course of the Syrian war.

At the time, Obama was criticized for ruining his country’s credibility by promising a military response and not delivering. More recently,  the deal has been criticized for being even more far-fetched  because Assad has continued  to mount chemical weapons attacks.

Back in 2013, then presidential aspirant Donald Trump roundly opposed Obama ordering air strikes against Syria in retaliation for using chemical weapons against civilians. Yet in April 2017, shortly after taking office, Trump ordered a missile strike against Assad’s regime after seeing the grisly images of gassed Syrian children.  Trump implicitly criticized Obama for not taking similar action while he was still in office.

While this moment has become the defining symbol for the lack of U.S. intervention in the war, a former Obama senior advisor and experts offered an intriguing explanation for why Obama never made good on his “red-line” threat: He was more concerned about not scuttling sensitive negotiations with Iran on an agreement to end their nuclear weapons program in return for the lifting of economic sanctions

Ben Rhodes, Obama’s foreign policy advisor, said in the documentary The Final Year that the Iran Nuclear Deal was the priority of Obama’s foreign policy strategy and influenced how the administration would proceed with Syria.

“If the U.S. had intervened more forcefully in Syria,” Rhodes said in the documentary that examines the last year of Obama’s presidency. “It would have dominated Obama’s second term and the [Iran Nuclear Deal] would have been impossible to achieve.”

Lee Smith, a senior fellow at the Hudson Institute, said that when the nuclear deal was signed by Iran, the U.S. and five other countries July 14, 2015, the Obama administration insisted that the Iran nuclear deal was independent of the administration’s Middle Eastern foreign policy.

However, Smith said that the negotiations were, in fact, directly affecting Obama’s foreign policy.

“Now former Obama officials have admitted what analysts had been arguing for several years: Obama cared only about getting a nuclear deal with Iran, no matter how that might affect other important US policies,” Smith said.

Along with Russia, the Iran government is one of Assad’s main ally’s and has helped the Syrian president regain his control over large portions of the country. Since the nuclear deal took effect, Iran has dramatically expanded its influence in Syria, supporting Assad’s military forces.

Smith argued that Iran’s position as an ally to Assad prevented the Obama  administration from intervening because of its desire to reach an agreement with the Iranian government on the nuclear deal.

However, not everyone agrees with Rhodes and Smith.

Peter Mandaville, an international relations professor at George Mason University and former State Department official during the Obama administration, insisted that considerations like geopolitics and security were “deliberately kept out of the picture” while the nuclear deal was being negotiated.

“I don’t believe that the Iran nuclear deal was a major consideration in the U.S. calculus not to intervene in Syria,” Mandaville said in an email. “I believe the Obama administration hesitated to intervene militarily in Syria because it determined that the level of commitment needed to change the behavior of the Assad regime would run a significant risk of pulling the United States into another protracted conflict in the Middle East.”

Since the war began, nearly 12 million people have fled or been displaced within the country, over 500,000 civilians have died, and, as of Tuesday, Assad retook the last rebel-held stronghold, effectively defeating the 2011 uprising.

The U.S. has been operating in Syria militarily since 2014. However, these operations have strictly been against ISIS forces in Syria and Iraq. While no military operations have been used against Assad with the exception of the bombing of an air field in response to the use of sarin gas, a nerve agent made from chlorine, the Trump administration has been outspoken against Assad in the past

Former-Secretary of State Rex Tillerson said in January that “A stable, unified, and independent Syria ultimately requires post-Assad leadership in order to be successful.”

“It is hard to predict what would have happened, if the U.S.  had intervened,” Paul Salem, the senior vice president for policy at the Middle East Institute, said about the Obama administration. “But, now five million refugees are outside of the country and hundreds of thousands of Syrians lost their lives.”