Washington ? The U.S. and Russia sealed the first major nuclear weapons treaty in nearly two decades Friday, agreeing to slash the former Cold War rivals’ warhead arsenals by nearly one-third and talking hopefully of eventually ridding a fearful world of nuclear arms altogether.

President Barack Obama said the pact was part of an effort to “reset” relations with Russia that have been badly frayed. And at home the agreement gave him the biggest foreign policy achievement of his presidency, just days after he signed the landmark health care overhaul that has been his domestic priority.

Obama and Russian President Dmitry Medvedev will sign the pact April 8 in Prague, where Obama gave a major speech on doing away with nuclear arms one year ago. The city is the capital of Czech Republic, a former Soviet satellite, now a NATO member.

If ratified by the Senate and by Russia’s legislature, the reductions still would leave both countries, by far the world’s largest nuclear powers, with immense arsenals — and the ability to easily annihilate each other. Together, the United States and Russia possess about 95 percent of the world’s nuclear weapons, according to the Center for Arms Control and Non-Proliferation.

Still, Obama called the pact a step toward “the peace and security of a world without nuclear weapons.” He said nuclear weapons “represent both the darkest days of the Cold War, and the most troubling threats of our time.”

Ratification in the Senate will require 67 votes, two-thirds of the senators.

Sen. Richard Lugar of Indiana, a leading Republican on the Senate Foreign Relations Committee, welcomed Friday’s announcement, saying he looked forward to receiving the treaty so that the committee could “work quickly to achieve ratification.”