How the Trump administration acts in the aftermath of the chemical attack in Syria’s Douma is significant for relations between the U.S. and Russia. If mismanaged, the two sides risk a deterioration in their relationship worse than experienced during the Cold War. Responding to the Douma attack with missiles, President Donald J. Trump has claimed “success.” It is too early for such celebration. In the coming days and months, we can expect an escalation of tensions between Washington and Moscow.