IT was THE most serious incursion into NATO territory since the foundation of the alliance in 1949. Nineteen Russian drones breached Poland’s airspace between 11.30pm on September 9th and 6.30am the next morning. Polish fighter jets, along with Dutch F-35s deployed to Poland earlier this month, shot down some of them. Others crashed. One fell 300km deep into Polish territory. Within hours Poland’s government invoked NATO’s Article 4, triggering immediate consultation with allies. The Polish army denounced it as an “act of aggression”. Donald Tusk, the country’s prime minister, noted that it was the first time Russian drones had been shot down over NATO territory. Poland, he said, was at its “closest to open conflict since the second world war”.