The United States does not want to implement a no-first-use nuclear policy, because the Pentagon believes such a stance would put into question Washington's willingness to protect its allies and its own vital interests, Trachtenberg told a hearing before the House Armed Services Committee on 28 March.
Trachtenberg said that the United States has for decades maintained a policy of "constructive ambiguity" regarding the employment of nuclear weapons, which has "deterried potential adversaries from nuclear coercion or aggression."
"A policy of ‘no-first-use' would undermine US extended deterrence and damage the health of our alliances because it would call into question the assurance that the United States would come to the defense of allies in extreme circumstances," he said, according to a transcript published on the committee's website.
"'No-first-use' is highly unlikely to be believed by our adversaries, but, even if it were, it is more likely to embolden them to test what they might perceive as weakened US resolve to defend our allies and vital interests with every means at our disposal than it is to promote peace," he added.
"A no-first use policy could undermine US nonproliferation objectives if allies and partners felt the need to develop or possess their own nuclear weapons to deter potential adversaries," Trachtenberg said.
Currently, among US allies, only France and the UK have nuclear weapons of their own. (Israel is widely believed to have nuclear weapons, but has not ever officially acknowledged them.) Belgium, Germany, Italy, Netherlands and Turkey are part of NATO's nuclear sharing program.
Under current policy, the US will employ nuclear weapons "only in extreme circumstance to defend the vital interests of the United States, allies and partners." Those "extreme circumstances" include "significant non-nuclear strategic attacks on US, allied or partner civilian population or infrastructure, nuclear forces, command and control or warning and attack assessment capabilities," Trachtenberg said, citing the Department of Defense's 2018 Nuclear Posture Review.
However, in the case of "significant non-nuclear strategic attacks," the US reserves the right to "make any adjustment in the assurance" to counter that threat, according to the 2018 Nuclear Posture Review.
The 2010 Posture Review threatened NPT-compliant countries with a "devastating conventional military response" in the case of a chemical or biological attack on the US or its allies. That clause has been removed in the updated version.