A decades-old massacre dispute is straining Poland and Ukraine's wartime alliance — and Russia is exploiting the rift to weaken support for Kyiv.

Poland, steeped in bad memories of the 1939 invasion and decades of Soviet dominance after World War II, is a firm backer of Ukraine’s defense against Russia. But a flare-up over…

As Poland observes a day of remembrance for the victims of a Ukrainian nationalist military unit, there are increasing signs that it could cut off its neighbor from military aid,…