‘Sanctions were not discussed at my meeting with President Putin,” Donald Trump tweeted Sunday. “Nothing will be done until the Ukrainian & Syrian problems are solved!” Hours before the two presidents met, Secretary of State Rex Tillerson underlined this tough line on sanctions by appointing Russia hawk Kurt Volker as chief U.S. envoy on Ukraine.

Messrs. Trump and Putin made limited progress on Syria, but Moscow has been intensifying its efforts to destabilize Ukraine. Mr. Putin has been reluctant to deploy large military...