A US B-52H strategic bomber at a South Korean Air Force base at Cheongju Airport in South Korea in support of longtime ally Seoul, in the face of North Korean military threats. Photograph: Anthony Wallace/AFP/Getty Last Monday night, the UN Security Council held a private meeting on nuclear non-proliferation. The French presidency warned that efforts to limit the spread of nuclear weapons are "facing increasingly serious challenges". The Nuclear Non-Proliferation Treaty (NPT) obliges nuclear states not to transfer nuclear weapons to other states, and commits others to not acquiring them. It is arguably Ireland's most important diplomatic achievement, coming into…