If the history of central and eastern Europe is being rewritten by the conflict in Ukraine, then so is the history of the north Atlantic alliance. Two people were killed on Tuesday evening in Polish territory, struck, it seems, by a Russian-made missile. The US president, Joe Biden, and the Warsaw government sought to dial down the tension, saying on Wednesday that the missile most probably came not from Russia but from Ukrainian air defence. The question for Poland, however, remains, as it would for any Nato member state, and especially one living in Russia’s shadow: what if this, or...