KYIV, Ukraine — Nobody can agree on an acceptable peace in Ukraine. Should Russia be allowed to keep the lands it took by force? Should Ukraine be allowed to join NATO? What sort of security guarantee is sufficient? What sort is reasonable? Deeper questions undergird these. What sort of future is possible for Ukraine? And what is America’s interest in this war, which could be seen as another flare-up of ancient Slavic intramural fighting? Pokrovsk, the current hot spot on Ukraine’s western front, is 5,500 miles from Peoria, Illinois. NATO membership or some other U.S. security guarantee is a pledge that boys from Peoria will die for Pokrovsk. This is a tough sell to an American public that is tired after 80 years as the global policeman. And would a ceasefire in this war really mean peace for Ukraine, as long as Russian President Vladimir Putin looms next door? It’s hard to imagine a less trustworthy neighbor. To make matters worse, many Ukrainians doubt that anything will get better after Putin. All these questions point us toward uncomfortable answers. The United States and Ukraine are allies, but our interests are not identical. There’s no panacea for the Eurasian steppe and the Black Sea. There’s no future where Ukraine lives safely and easily. But there is a future where Russia is more afraid to aggress on Ukraine, and this future does involve the U.S., and some...