In the past 20 years most countries with nuclear ambitions have been geopolitical minnows, like Libya and Syria. In the next decade the threat is likely to include economic and diplomatic heavyweights whose ambitions would be harder to restrain. China’s rapidly increasing regional dominance and North Korea’s growing nuclear arsenal haunt South Korea and Japan, two of Asia’s largest powers. Iran’s belligerence and its nuclear programme loom over the likes of Saudi Arabia and Turkey (see article). Proliferation is not a chain reaction, but it is contagious. Once the restraints start to weaken they can fail rapidly.