It’s obvious — if not necessarily to the Biden administration — that the success of a nation’s foreign policy depends, in part, on its ability to shape the way in which it is seen abroad. The U.S. was able to ensure the survival of West Berlin — a highly vulnerable exclave — throughout the Cold War in no small part because the Kremlin could never be certain how fiercely America would react if the Soviets attempted to take over the western half of that divided city by force. The Berlin Airlift was an early and remarkable display of support, but the dispatch at the same time of three B-29 bomber groups, which may or may not have been in a position to initiate a nuclear strike, to the U.K. may also have helped persuade Stalin to take things no further. To work, deterrence must be credible. The American, British, and French troops stationed in West Berlin for decades were no more than a tripwire, but Moscow could only speculate about what would follow from triggering it. It never took the risk.