In the Sunday reading from the Book of Revelation, “a voice as loud as a trumpet” says to John, “I hold the keys to death and the netherworld.” Expressing grave concern over the potential for nuclear proliferation and “nuclear terrorism,” President Barack Obama Tuesday announced a fundamental shift in the nation’s nuclear strategy.

Arguing that the spread of atomic weapons to rogue states or terrorists represents a worse threat than the nuclear Armageddon feared during the Cold War, Obama said his administration would narrow the circumstances in which the U.S. might launch a nuclear strike, forgo the development of new nuclear warheads, and seek even deeper reductions in U.S. and Russian arsenals.

"The greatest threat to U.S. and global security is no longer a nuclear exchange between nations, but nuclear terrorism by violent extremists and nuclear proliferation to an increasing number of states," Obama said.

The announcement set the stage for his trip to Prague Thursday to sign a new arms reduction agreement with Russia to cut stockpiles of long-range nuclear warheads by a third, as well as for a "nuclear summit" of 40 world leaders Obama will host next Monday in Washington. Those gathered will discuss improving safeguards against terrorists acquiring nuclear bombs.

From the start of his term in office Obama has put halting the spread of atomic arms near the top of his defense priorities. But during his first year he failed to achieve a significant breakthrough on arguably the two biggest threats: Iran and North Korea. "Given al-Qaeda's continued quest for nuclear weapons, Iran's ongoing nuclear efforts, and North Korea's proliferation, this focus is appropriate and, indeed, an essential change from previous" policy, U.S. Defense Secretary Robert Gates said Tuesday.

While the new policy rules out new nuclear weapons or testing, it calls for setting aside billions of dollars to "modernize" existing U.S. weaponry. The new strategy, though a major shift, isn't as far-reaching as liberals had hoped nor as radical as conservatives had feared, said analysts.

It contains initiatives sought by both sides, including those of Republican senators whose votes Obama will need in order for the new arms reduction treaty with Russia to be ratified.

Articles by Dan De Luce for Agence France Presse, Jonathan S. Landay and Margaret Talev for McClatchy Newspapers, and Robert Burns and Anne Flaherty for Associated Press