The first reason is that it would take the world one small but significant step towards Barack Obama’s goal of abolishing nuclear weapons. A NFU declaration implies that the only purpose of nuclear weapons is to deter — through the threat of retaliation — their use by others. Then if no one else has them, no one needs them — making the argument for abolition a bit easier to win. Abolishing nuclear weapons, if it could ever be done, would make the world a safer place.

The second reason to support a NFU declaration is that it would simply recognise reality. The United States’ threat to start a nuclear war, which a NFU declaration would nullify, is simply no longer credible in any conflict with a nuclear-armed adversary. The risk of nuclear retaliation against the United States itself is too costly. And nuclear-armed adversaries are the only ones that matter.

It was different during the Cold War, when the United States held off massive Soviet conventional forces by threatening to use nuclear weapons against them. The threat was credible because Washington was plainly willing to accept the risk of a nuclear attack on US soil to stop the Soviets overrunning Western Europe.

But today’s geopolitical rivalries are nothing like the Cold War; it is far less credible that US leaders would accept the horrific costs of a nuclear attack on their homeland over a dispute in the East China Sea or the Baltics. So the nuclear ‘first use’ option for the United States is now an illusion and a bad basis for strategic policy. Obama would be wise to ditch it by making a NFU declaration.

But a NFU declaration would have some unintended and unwelcome consequences which should not be ignored. This is the thought I was developing in the earlier post which Gareth Evans was responding to. The most important of these unwelcome consequences is the effect on the confidence of US allies in Washington’s commitment to their security.

A ‘first use’ option by the United States may be illusory, but it is still taken very seriously by US allies like Poland and Japan that confront large and threatening neighbours like Russia and China. They believe that the United States’ threat to cross the nuclear threshold to defeat an overwhelming conventional attack on its allies is vital to their security, just as it was during the Cold War.

They cling to the illusion that the United States is still willing to accept the huge risks and costs involved in starting a nuclear war, because its commitment to its allies is as strong as ever. No one in Washington has so far been prepared to disabuse them of this, but a NFU declaration would smash this illusion and leave them feeling much more exposed. That would have unsettling consequences as they looked for other ways to ensure their security.

Supporters of a NFU declaration like Gareth Evans, Ramesh Thakur and others dismiss this concern. They say that a NFU declaration will have no effect on the credibility of US alliance commitments, because the United States’ conventional forces are now so preponderant that they can deter or defeat any adversary without resorting to nuclear weapons. But all the evidence points the other way.

Certainly, the United States spends more on its conventional forces than any other country and it has an unmatched capacity to deploy those forces globally. But that doesn’t mean it can outmatch rival powers on their home turf. On the contrary, as key recent studies have shown, the United States would quickly be overpowered by Russian conventional forces in the Baltic States and would face a costly and escalating stalemate against China in a maritime war in Asia.

Optimists in the United States hope new technological breakthroughs, packaged as the ‘third offset strategy’, will swiftly restore US conventional military preponderance, but that is so far just a three-word slogan and a pious hope. It is much more likely that the conventional military balance in their respective theatres will keep shifting Russia’s and China’s way.

US allies like Japan understand this reality all too well. If a NFU declaration smashes the illusion that the United States would be willing to use nuclear forces to defend them from conventional attack, their confidence in the alliance will plummet — then they will start looking for alternatives. For Japan, at least, that could mean building nuclear weapons of its own. Or it could push Japan in the other direction, towards passive acceptance of China’s regional primacy. Either would be very unsettling.

Advocates of a NFU stance need to weigh these potential consequences. In Asia, a US declaration could spur nuclear proliferation, or promote a new China-led regional order. These possibilities might not be enough to swing the argument against a NFU declaration, if only because they may well happen anyway as it becomes clearer that the United States’ ‘first use’ option is an illusion. But a NFU declaration could well bring them about faster, and foreclose other, less scary outcomes. This needs at least to be acknowledged.

Hugh White AO is Professor of Strategic Studies at The Australian National University. His work focuses primarily on Australian strategic and defence policy.