Mr. Putin’s actions have triggered massive sanctions, with more to come if he launches a full-scale assault and attempts to seize the entire country. These would devastate not just his country’s economy but also his tight circle of corrupt cronies — who in turn could challenge his leadership. What is sure to be a bloody and catastrophic war will drain Russian resources and cost Russian lives — while creating an urgent incentive for Europe to slash its dangerous reliance on Russian energy. ... Such an act of aggression would almost certainly drive NATO to significantly reinforce its eastern flank and to consider permanently stationing forces in the Baltic States, Poland and Romania. ... And it would generate fierce Ukrainian armed resistance, with strong support from the West.