A Russian incursion into Ukraine could, in a perverse way, save the current European order. NATO would have no choice but to respond assertively, bringing in stiff sanctions and acting in decisive unity. By hardening the conflict, Mr. Putin could cohere his opponents. Holding back, by contrast, could have the opposite effect: The policy of maximum pressure, short of an invasion, may end up dividing and paralyzing NATO.

To see how that might play out, we need only look to Germany. Before the crisis, Germany was America’s closest ally in Europe, boasted a special relationship with Moscow and was the most important partner for Eastern and Central Europe. Today, some in Washington have questioned the country’s willingness to confront Russia, Berlin’s relationship with Moscow is fast deteriorating, and many Eastern Europeans are agitated by Germany’s apparent reluctance to come to their support. Germany’s difficulties are a hint of what could come if Mr. Putin continues his brinkmanship, without providing the certainty of an actual invasion.

Germany, crucially, has not changed — but the world in which it acts has. (The country is “like a train that stands still after the railway station has caught fire,” Bojan Pancevski, The Wall Street Journal’s Germany correspondent, told me.) Today, geopolitical strength is determined not by how much economic power you can wield but by how much pain you can endure.