Earlier this month, the Organization for Security and Cooperation in Europe reported that rebels, well-known to be Russia-backed, moved heavy weaponry, including tanks and other military vehicles, into the western Ukrainian town of Maryinka ahead of a major assault. The European Union later called the ensuing deadly attack the most serious violation of the Minsk cease-fire since it was signed in February.

As Vladimir Putin’s bold and bald aggression continues unabated even in the face of a signed cease-fire, it can’t be ruled...