In the middle of the last century, as the United States and Russia rapidly amassed thousands of nuclear weapons, China stayed out of the arms race, focusing its energy on growing its economy and broadening its regional influence.

Beijing did build hundreds of nuclear weapons during those years, but the nation’s leaders insisted their modest arsenal was merely for self-defense. Since China’s first nuclear weapons test, in 1964, the country has pledged loudly to never go first in a nuclear conflict — no matter what. That stance, coupled with a stated strategy of “minimum” deterrence, didn’t demand the level of American fear, loathing and attention that the Russian threat did.

Now there is increasing unease in Washington about China’s nuclear ambitions. The Pentagon says Beijing is on track to double the number of its nuclear warheads by the decade’s end, to 1,000 from 500 — a development that senior U.S. officials have publicly called “unprecedented” and “breathtaking.” China has drastically expanded its nuclear testing facility and continued work on three new missile fields in the country’s north, where more than 300 intercontinental ballistic missile silos have recently been constructed.