A Russian missile crashed in Poland on Thursday during an overnight assault on neighboring Ukraine, NATO said, an event that could risk bringing Russia closer to direct confrontation with the Western military alliance, to which Poland belongs. Mark Rutte, the NATO secretary general, said the incident was a “violation of Poland’s airspace by a Russian missile” and vowed to strengthen alliance defenses. The missile — which the Polish authorities originally called an “unidentified object” — struck a field in southeastern Poland, causing no injuries. On Thursday morning, Prime Minister Donald Tusk of Poland told an emergency meeting that the missile...