President Trump’s acquiescence to Turkey’s move to send troops deep inside Syrian territory has in only one week’s time turned into a bloody carnage, forced the abandonment of a successful five-year-long American project to keep the peace on a volatile border, and given an unanticipated victory to four American adversaries: Russia, Iran, the Syrian government and the Islamic State. Rarely has a presidential decision resulted so immediately in what his own party leaders have described as disastrous consequences for American allies and interests. How this decision happened — springing from an “off-script moment” with President Recep Tayyip Erdogan of Turkey,...