The U.S. missile strike on Shayrat Airbase on 7 April 2017 was the first time the U.S. deliberately attacked Syrian government forces, and marked the start of a series of direct military actions by U.S. forces against the Syrian government and its allies that occurred during the periods of May–June 2017 and February 2018. In mid-January 2018, the Trump administration indicated its intention to maintain an open-ended military presence in Syria to counter Iran's influence and oust Syrian president Bashar al-Assad. In early September 2018, the U.S. began implementing a new strategy that sought to indefinitely extend its military effort, launching a major diplomatic push to achieve American objectives in Syria. However, on 19 December, President Trump unilaterally ordered the withdrawal of the 2,000–2,500 American ground troops in Syria, which was initially set to take place in a 90-day period, and to be completed in 2019. With proliferating concerns over a potential power vacuum, the U.S. announced on 22 February 2019 that instead of a total withdrawal, a contingency force of around 400 American troops would remain garrisoned in Syria indefinitely, and that their withdrawal would be gradual and conditions-based, marking a return to a policy of open-ended American military presence in the country. By the end of 2018, the SDF, assisted by the coalition, had liberated over 20,000 square kilometers of territory, and three million Syrian civilians from the Islamic State.