Barnett opposed British participation in the Kosovo War of 1999, arguing that Yugoslavia was "a sovereign state committing no aggression beyond its own borders, military action against it is a breach of the UN Charter and likewise of the North Atlantic Treaty". Furthermore, on 30 March 1999 he claimed that the war's course had vindicated his original stance on "Nato's ill-thought-out policy, based on emotion and simplistic moralising...In particular, it has plunged the Kosovans, the objects of Nato's solicitude, into their present calamity". Later that year Barnett returned to the subject, saying that the 80-day-long air campaign against Serbian forces demonstrated "that air power is a clumsy means of political coercion" and "that Bosnia should have served as a warning to us not to get entangled over Kosovo, and that if we did get entangled, we would finish up to our necks in trouble – which we have".