US President Barack Obama unveiled a much-anticipated nuclear defense policy on Thursday, following through – with notable caveats – on promises to scale back the role of atomic weapons (Washington Post).

More than a year in the making, the Nuclear Posture Review represents a delicate negotiation within the administration. It comes out two days before the United States is scheduled to sign a new arms reduction treaty with Russia and nearly a week before Obama’s nuclear security summit in Washington. And all of this precedes a major review conference under the Nuclear Non-proliferation Treaty in May.

Nobody is entirely happy with the document, but most non-proliferation advocates consider it a step in the right direction. “The bottom line is that this review opens the door to truly transformational changes in US nuclear policy, but does not go through it,” says Stephen Young, who tracks the issue for the Union of Concerned Scientists in Washington.

The document says the US will not use or threaten to use nuclear weapons against a non-nuclear state – so long as that state is party to and in alignment with the nonproliferation treaty. That language carves out a huge exception for, say, Iran or North Korea. But the new policy (largely) excludes nuclear weapons as a deterrent against biological and chemical weapons.

And whereas the previous administration had pursued work on a new “reliable replacement warhead,” the new policy states that the United States will not pursue new nuclear arms. “Lifetime extension programs” will use existing designs and will not provide new military capabilities. But again, the administration left itself some wiggle room: Replacement of core nuclear components – those parts which go flash-bang upon detonation – would take place only if critical goals “could not otherwise be met, and if specifically authorized by the President and approved by Congress.”

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The document anticipates the United States’ agreement with Russia – which would bring the US arsenal down to about 1,550 deployed strategic warheads – but does not paint a clear picture on how to get down to a minimal arsenal, says Matthew Bunn, a nuclear expert at Harvard University. “I would have liked to see a more forthright discussion on going down to lower levels,” he says.

Perhaps the biggest question going forward is how the directors of the Energy Department’s core nuclear weapons laboratories – Los Alamos and Sandia national laboratories in New Mexico and Lawrence Livermore national laboratory in California – will respond. Earlier this year they penned letters to a House Armed Services subcommittee objecting to the primary conclusion – though not necessarily the detailed findings – of an assessment by the expert JASON panel indicating that the nation’s nuclear stockpile can be maintained for decades into the future with existing programs.

The administration is planning a push to ratify the Comprehensive Test Ban Treaty, and that kind of opposition from the lab directors will certainly complicate the effort. But UCS’ Young points out that Obama is working to please the lab directors in other areas, notably by increasing budgets, expanding facilities and making it clear that current scientists and future recruits have a bright future ahead.