Mr. Putin is also grappling with the arrival of a new world of unbridled American power under President Trump, which is checking Russia’s global influence and ripping up Moscow’s playbook for partnerships abroad. For years, Mr. Putin supported anti-American authoritarian governments in Iran, Venezuela and Cuba, with little worry that Washington would use its overwhelming military power to kill, capture or push out their leaders. That has now changed, as Mr. Trump has demonstrated a willingness to disregard international norms and engage in foreign adventurism by fully exploiting Washington’s might. The last two months have brought the U.S.-Israeli killing of...