WASHINGTON — President Trump on Thursday defended his plan to withdraw American troops from Syria against mounting criticism from Republicans and Democrats and concerns from his military and diplomatic advisers who were caught off guard by his abrupt announcement a day earlier.

“Getting out of Syria was no surprise,” Mr. Trump wrote in one of a series of Twitter posts on Thursday morning. He presented a string of justifications for his decision, including supportive quotes from a few who applauded his agenda.

The president also pushed back against reports — which he derided as “Fake News” — that the American military exit from Syria would advance Russian and Iranian foreign policy objectives. In Moscow, President Vladimir V. Putin of Russia said he supported the plan and called it “the right decision.”