PHILADELPHIA — For all its horrific power, the atom bomb — leveler of Hiroshima and instant killer of some 80,000 people — is but a pale cousin compared to another product of American ingenuity: the hydrogen bomb.

The weapon easily packs the punch of a thousand Hiroshimas, an unthinkable range of destruction that lay behind the Cold War’s fear of mutual annihilation. It was developed in great secrecy, and Washington for decades has done everything in its power to keep the details of its design out of the public domain.

Now, a physicist who helped devise the weapon more than half a century ago has defied a federal order to cut from his new book material that the government says teems with thermonuclear secrets.

The author, Kenneth W. Ford, 88, spent his career in academia and has not worked on weapons since 1953. His memoir, “Building the H Bomb: A Personal History,” is his 10th book. The others are physics texts, elucidations of popular science and a reminiscence on flying small planes.