On 6 April 2017, the United States launched about sixty Tomahawk missiles at what is reportedly a military airfield.  The Trump administration said that the strike was directed at the airplanes that were used to carry out a chemical attack:

“Assad choked out the helpless,” the President said. “There can be no dispute that Syria used banned chemical weapons … and ignored the urging of the UN Security Council,” he said during short remarks to reporters at Mar-a-Lago.

“The refugee crisis continues to deepen and the region continues to destabilize.”

The Pentagon said in a statement that the U.S. warned Russian forces in advance in order to minimize casualties:

As always, the U.S. took extraordinary measures to avoid civilian casualties and to comply with the Law of Armed Conflict.  Every precaution was taken to execute this strike with minimal risk to personnel at the airfield.
 
The strike was a proportional response to Assad’s heinous act. Shayrat Airfield was used to store chemical weapons and Syrian air forces.  The U.S. intelligence community assesses that aircraft from Shayrat conducted the chemical weapons attack on April 4.  The strike was intended to deter the regime from using chemical weapons again. 
 
Russian forces were notified in advance of the strike using the established deconfliction line.  U.S. military planners took precautions to minimize risk to Russian or Syrian personnel located at the airfield. 

 

The strikes came a day after Syria launched chemical attacks that killed dozens of civilians in the province of Idlib. It was the latest in an ongoing civil war that has killed thousands and displaced millions in the worst refugee crisis in decades:

The attack may put hundreds of American troops now stationed in Syria in greater danger. They are advising local forces in advance of a major assault on the Syrian city of Raqqa, the Islamic State’s de facto capital.

The decision to strike follows 48 hours of intense deliberations by U.S. officials, and represents a significant break with the previous administration’s reluctance to wade militarily into the Syrian civil war and shift any focus from the campaign against the Islamic State.

Photographs of children writhing in pain and frothing at the mouth that emerged from the 4 April 2017 nerve gas attacks are said to have galvanized the United States into action:

Their symptoms – foaming at the mouth and fluid filling the lungs, which can lead to suffocation – were consistent with the effects of Sarin gas, he said. At least 74 people died in the alleged chemical attack, according to a document detailing the victims’ names, released by the Idlib Health Directorate.

World leaders accused Syrian president Bashar al-Assad’s government of carrying out the attack. However, Damascus issued a statement categorically denying that it used any form of poisonous gas in Khan Sheikhoun.

The airstrikes represent an abrupt shift in policy by the United States, which until the end of March 2017 appeared to hold a relatively noninterventionist view toward Syria:

“You pick and choose your battles and when we’re looking at this, it’s about changing up priorities and our priority is no longer to sit there and focus on getting Assad out,” U.S. Ambassador Nikki Haley told a small group of reporters.

“Do we think he’s a hindrance? Yes. Are we going to sit there and focus on getting him out? No,” she said. “What we are going to focus on is putting the pressure in there so that we can start to make a change in Syria.”

In Ankara on Thursday, U.S. Secretary of State Rex Tillerson said Assad’s longer-term status “will be decided by the Syrian people.”

According to the United Nations, there have previously been more than 160 chemical attacks in Syria since the country’s civil war began in 2011.  In August 2013, another devastating attack killed hundreds of people (by some reports more than a thousand) around Damascus. 

Sources:

Bradford, Alexandra.     “The Aftermath of an Alleged Chemical Weapon Attack in Idlib.”
    Syria Deeply.    5 April 2017.

Stewart, Phil and Holland, Steve.    “Trump Says He Ordered Military Strikes Against Assad Airbase in Syria.”
  Reuters.    6 April 2017.

  “US Launches Cruise Missiles at Syria.”   Al Jazeera.   6 April 2017.