The Russia-Ukraine war will likely end in one of three ways: Russia will annex all of Ukraine and reincorporate it into Russia; Russia will install a puppet regime in Kyiv and exercise effective political control over Ukraine's foreign policy; or Russia will militarily occupy parts of Ukraine and suffer the resistance of an insurgency that may be armed and supported by outside powers. Regardless of which scenario plays out, this is and will be a tragic and unnecessary war — entirely preventable, if only the Europeans and Americans had heeded the warnings of George F. Kennan about the likely unintended...