CAMBRIDGE – When the Soviet Union collapsed in 1991, Ukraine inherited part of its nuclear arsenal. But in the 1994 Budapest Memorandum, Ukraine agreed to return these weapons to Russia in exchange for “assurances” from Russia, the United Kingdom, and the United States that its sovereignty and borders would be respected. Russia brazenly violated this promise when it annexed Crimea in 2014, and tore up the Memorandum with its full-scale invasion of Ukraine on February 24. Many observers have concluded that Ukraine made a fateful mistake by agreeing to surrender its nuclear arsenal (once the world’s third largest). Are they right?

In the early 1960s, US President John F Kennedy predicted that at least 25 states would have nuclear weapons by the following decade. But in 1968, United Nations member states agreed to a non-proliferation treaty that restricted nuclear weapons to the five states that already had them (the US, the Soviet Union, Britain, France, and China). Today, just nine states have them – the five named in treaty signatories plus Israel, India, Pakistan, and North Korea – but there are more “threshold states” (countries with the technological ability to build nuclear weapons quickly) considering the option.

Some analysts suggest that proliferation might be a good thing, because a world of nuclear-armed porcupines would be more stable than a world of nuclear wolves and unarmed rabbits. In their view, Russia would not have dared to invade a nuclear-armed Ukraine. Moreover, they question why some states should have a right to nuclear weapons while others do not.