Defeat for Ukraine would be a global disaster. Nato must finally step in to stop Russia

The pressing question of Ukraine's membership is expected to dominate this week's pivotal summit of 31 Nato states in Vilnius, Lithuania's capital. Volodymyr Zelenskiy knows a gold-embossed official invite is unlikely to arrive while his country is still at war with Russia. But Ukraine's leader is urging US president Joe Biden and alliance leaders to take immediate "concrete steps", including security guarantees, a roadmap and timeline. Zelenskiy argues that would boost morale and send an implacable message to Vladimir Putin. Zelenskiy is right. Like Finland, Ukraine's accession should be fast-tracked. Yet important though this issue is, Nato faces a far…