It was bracing that Ukraine launched its counteroffensive against Russian invaders as we celebrate the anniversary of the 1944 D-Day landings this week. This assault could turn the tide of the battle for Ukraine, just as the Allied assault on the Normandy beaches altered the trajectory of World War II. Military campaigns are rarely all or nothing, but this one comes close. If Ukraine can drive back an already shaky Russian army, it stands a chance of forcing Moscow to bargain for an end of its failed invasion. But if Ukraine fails, it would be a bitter blow to the...