This week, dignitaries and Western military veterans celebrated the 70th anniversary of the Berlin airlift, the mammoth yearlong effort to break a ground blockade by the Soviet Union. Whether the United States and its allies would go to the same length to support an ally today, however, remains a very open question.

That, at least, appears the growing belief in Moscow and Beijing, both intent on pressuring their neighbors who are also U.S. allies. It's a perception the U.S. military is going to increasing lengths to counter, despite — or perhaps in some respects because of — questions over where U.S. President Donald Trump himself stands. That uncertainty, however, risks opening the door to ever more dangerous confrontations.

The question goes to the heart of the always messy paradox of deterrence. The readier countries are to band together against an aggressive but powerful state determined to get its way, the less likely such a conflict is to happen. But it also points to a way in which the world has changed. Even the most powerful nations, whether U.S. allies or adversaries, are increasingly tied into a globalized supply chain beyond anyone's control. That makes Berlin-style blockades much harder to envisage — but it also means nations lack the military or other resources to get around them if they happen.