Read James Mattis’ letter to Trump resigning as Defense secretary

Mattis’ resignation comes on the heels of Trump’s controversial plan to withdraw U.S. troops from Syria.

The president’s decision, announced on Wednesday, will reportedly take more than 2,000 U.S. servicemembers out of the country, ending the ground strategy against the Islamic State. Trump said in a tweet Wednesday morning that “we have defeated ISIS in Syria, my only reason for being there during the Trump Presidency.”

Neither the White House nor The Pentagon immediately responded to CNBC’s requests for comment on the president’s announcement.

Read Mattis’ full letter to the president below: