Game, set and match to Vladimir Putin in Syria. President Trump’s hasty and ill-advised withdrawal of American tripwire forces from Syria has forced the Kurds into a deal with dictator Bashar Assad, whose forces are re-entering the country’s Northeast region. With a modest investment, the Russian president not only staved off defeat for Mr. Assad but has moved close to his goal of reunifying Syria under his client’s control.

Iran’s Supreme Leader Ali Khamenei is also taking a victory lap. With the collapse of America’s position...