All it takes is indifference by some countries in the EU. If Poland or Hungary or Czech Republic hesitates and demurs about taking some action against a little putsch by Putin into Latvia, then the foot gets in the door and there’s no stopping. Putin’s a judoist. He understands leverage. The best advantage a judo fighter wants is hesitation by the opponent. You may resist or move with the action, but being indifferent gives the initial attacker an extra move, so to speak.