Previous US administrations were reluctant to adopt legislation that would legally bind Washington to only use its nuclear weapons in response to a nuclear attack. The new bill is likely to meet opposition as well.

Senator Elizabeth Warren (D-MA) and House Armed Services Committee Chairman Adam Smith (D-WA) have presented legislation called "The No First Use Act," which aims to prevent the US from using its nuclear arsenal first in case of an armed conflict.

According to the language of the bill, the US would only be allowed to use its nuclear arsenal if attacked with nuclear weapons by an adversary. Under the current administration's Nuclear Posture Review, the US reserves the right to use nuclear weapons in case of a "significant" non-nuclear attack on the US itself, its allies or its nuclear infrastructure.

"Our current nuclear strategy is not just outdated — it is dangerous," Smith and Warren said in a joint statement. "By making clear that deterrence is the sole purpose of our arsenal, this bill would reduce the chances of a nuclear miscalculation and help us maintain our moral and diplomatic leadership in the world."

Previous US administrations have been unwilling to accept such legislation, and Defense News speculates that the current Republican-controlled Senate is also unlikely to accept it. However, Smith, being the Armed Services Committee chair, might use his position to elevate the language into the annual defense policy bill, the 2020 National Defense Authorization Act; doing so might spark resistance from key Republicans, who have been supportive of Trump's current nuclear stance, Defense News says. 

Former President Barack Obama, a Democrat, was also reluctant to adopt such a policy, saying it could "embolden" nations such as Russia, China or North Korea to attack US allies. This, according to Obama, would undermine US allies' confidence in American commitment to their defence, which, in turn, would encourage them to seek their own nuclear deterrence.

The Union of Concerned Scientists, however, welcomed the new bill in a Wednesday statement.

The organization said that passing the bill would "reduce the risk of miscalculation during a crisis with Russia, China or North Korea; strengthen the Nuclear Nonproliferation Treaty by demonstrating the United States is serious about reducing the role of nuclear weapons in its security policy, and reduce risks associated with the president's sole authority to order the use of nuclear weapons by removing the option of using them first."

Stephen Young, Washington representative for the UCS Global Security Program, also hailed the idea, according to Defense News.

"The purpose of US nuclear weapons is to deter a nuclear attack on the United States and its allies. Any other purpose simply makes nuclear war more likely," he said.

Presented on January 17, the Pentagon Missile Defense Review explicitly singled out Russia and China as primary possible adversaries of Washington that might launch a nuclear offensive at American soil. Iran and North Korea, which were in the spotlight in earlier versions of the report, have now been pushed to second place, according to a review by Foreign Policy.