The presence of U.S. troops in northern Syria was designed to protect America’s allies and keep its enemies there in check. President Trump’s unilateral withdrawal from the region quickly, and predictably, unraveled a tenuous peace on the volatile border between Syria and Turkey. His decision handed a gift to four American adversaries: Iran, Russia, the Syrian government and the Islamic State. David E. Sanger of The Times explains why “the worst-case scenario is even worse than you can imagine.” Guest: David E. Sanger, a national security correspondent and a senior writer at The New York Times. For more information on today’s episode, visit nytimes.com/thedaily.  Background coverage:President Trump lashed out in defense of his decision to remove U.S. troops from northeastern Syria in response to rare bipartisan condemnation from Congress.Russian troops have already occupied abandoned American outposts in Syria as Moscow moves to fill the power vacuum.“Don't be a fool! I will call you later.” Read the letter President Trump sent to Turkey’s leader.