INTERPRETING any country’s pronouncements about its nuclear weapons can be a study in fine distinctions, but occasionally a state says — or fails to say — something in a clear break from the past. A Chinese white paper on defense, released on Tuesday, falls into this category and now demands our attention, because it omits a promise that China will never use nuclear weapons first.

That explicit pledge had been the cornerstone of Beijing’s stated nuclear policy for the last half-century. The white paper, however, introduces ambiguity. It endorses the use of nuclear weapons in response to a nuclear attack but does not rule out other uses.

With North Korea making overt nuclear threats, the job of deciphering Beijing’s cryptic and mild-sounding statement may not seem a priority. Indeed, it is because the likelihood of nuclear escalation with China is low that most defense experts are likely to focus instead on what the white paper has to say about China’s rapidly expanding conventional military capabilities.

But all of those developments may be closely connected.

In 1964, immediately after testing its first nuclear weapon, China promised to “never at any time or under any circumstances be the first to use nuclear weapons.” This “no-first-use pledge” was explicitly and unconditionally included in each of China’s defense white papers, from the first, in 1998, through the sixth and most recent, in 2011. It was among the strongest assurances in the world of no-first-use, a stance that the United States has never taken.