Is China Really Threatening the U.S. With Nuclear Weapons?

China, according to the Federation of American Scientists, has 270 warheads in its nuclear arsenal.

The Washington-based research group's estimate has never been challenged by the Pentagon. It compares with an official tally of 4,480 nuclear warheads for the US. Unlike the American side, China also renounces “first use” of nuclear weapons and holds that its ability to retaliate is sufficient to deter attack.

Why, then, is Beijing's modernization of its nuclear arsenal — something that Washington is also doing — considered a major security threat requiring a sharp turn in US policy regarding the use of nuclear weapons?