WASHINGTON — The Russian military intervention in Ukraine’s autonomous republic of Crimea has brought relations between the United States and Russia to their lowest level in a quarter century. It has transgressed the sovereignty of one of the most populous countries in Europe, violated the terms of a diplomatic agreement to respect Ukraine’s borders, and placed Russia on a war footing with one of the few states in the post-Soviet world that has managed to hold multiple free elections. It is a military operation that is unsanctioned by any international body, wholly open-ended, and blessed only by the Russian Parliament.

Crimea is routinely described as “pro-Russian,” given that an estimated 58 percent of the population of two million is ethnic Russian, with another 24 percent Ukrainian and 12 percent Crimean Tatar. Many of its inhabitants, regardless of ethnicity, are actually Russian citizens or dual-passport holders. But the picture is even more complicated. A vital naval base run by another country, a community of patriotic military retirees, a multiethnic patchwork, a weak state and competing national mythologies — that mixture is why a Crimean conflict has long been the nightmare scenario in the former Soviet Union and now represents the gravest crisis in Europe since the end of the Cold War.

But that is also the reason all sides must tread carefully. Affirmations about territorial integrity and cries of foreign invasion are empty mantras at a moment when a major European country — unbuilt by a string of fatuous governments and now further destabilized from abroad — has ceased to exist as a functionally unified state. NATO cannot possibly extend security guarantees to a government that does not control its own territory. Yet even in the midst of a standoff, Russia and the West have a clear common interest: forestalling a civil war in the heart of Europe....