Backgrounder: The Ain al-Asad military base

Ain al-Asad is an air base located in Anbar province of western Iraq used by U.S. and Iraqi armed forces.

The air base was first used by American forces after the 2003 U.S.-led invasion, and American troops were stationed there amid the fight against ISIL in Iraq and Syria. 

It houses about 1,500 U.S. and coalition forces, according to AP. An estimated 5,000 U.S. troops are stationed throughout Iraq.

Two years ago, Trump made a surprise Christmas visit to the base, one of the largest U.S. military bases in Iraq. In a speech to U.S. troops, he said that the U.S. would maintain its presence in Iraq and "watch over Iran."

U.S. Vice President Mike Pence paid an unannounced visit to the air base on November 23.

Read more: 

Trump wants U.S. military in Iraq to 'watch Iran'

Pence reassures Kurds of U.S. support in surprise visit to Iraq