The inclusion of a phrase outlawing the threat of using nuclear arms in the world's first-ever treaty to ban such weapons gives the accord teeth — but also makes it harder for allies of nuclear-armed countries to support the pact.

"Nuclear umbrella states" — countries like Japan and the Netherlands that receive an extended security deterrent from their nuclear allies — have so far indicated no willingness to join the treaty, which was adopted July 7 by the United Nations, though activists hold out hope this will change someday.

"Ever since the U.S. bombed the two cities in Japan in 1945, nuclear weapons have been central to military postures," John Burroughs, executive director of the International Association of Lawyers against Nuclear Arms, said on the sidelines of the conference that ended with the treaty's historic adoption.