In 1981 Kenneth Waltz published a controversial Adelphi Paper, 'The Spread of Nuclear Weapons: More May Be Better', in which he turned the conventional wisdom on its head by arguing that the spread of nuclear weapons would not be a terrifying prospect. This article rejects the proposition that fear of nuclear destruction can serve as a permanent basis of international order, and argues that securing order depends upon the building of trust between nuclear-armed and arming powers. A key contribution here has been the theory and practice of security communities, which opens up the promise of replacing nuclear threats by a new international politics in which force has been delegitimated as an instrument of state policy. This article discusses the potential for nuclear trust-building through the example of the security community that developed between Argentina and Brazil in the 1980s. Both countries had the potential to develop nuclear weapons by the end of the 1970s, and there were concerns that their rivalry might lead to a regional nuclear arms race. Having explored the factors that promoted trust between Buenos Aires and Brasilia, the article considers the lessons that can be learned for nuclear trust-building elsewhere.