Replacing Trident would breach customary international law, in particular because it would infringe the intrangressible requirement that a distinction must be drawn between combatants and non-combatants. If the envisaged use of force is itself unlawful, the stated readiness to use it would be a threat prohibited under Article 2, paragraph 4 of the UN Charter and that the replacement of Trident would be likely to constitute a material breach of article VI of the Nuclear Non-Proliferation Treaty. If the UK develops its nuclear provision, it will be tearing-up the Non Proliferation Treaty, having abandoned any vestige of commitment to it. Where will that leave the prospect of negotiating away existing nuclear weapons? Where will that leave the prospect of persuading countries without nuclear weapons not to acquire them? How can we persuade Iran to respect the NPT when we do not? The use, the threat of use, the planned replacement of Trident is illegal. The Scottish Parliament should take the opportunity today to send a clear message to Whitehall that Scotland respects the rule of law and expects Downing Street to do so also.