U.S. President Barack Obama and Russian President Dmitry Medvedev sealed a landmark arms-control treaty on Friday to slash their countries’ nuclear arsenals by a third and will sign it on April 8 in Prague.

After months of deadlock and delay, a breakthrough deal on a replacement for the Cold War-era START pact marked Obama’s most significant foreign policy achievement since taking office and also bolsters his effort to “reset” ties with Moscow.

Obama and Medvedev put the finishing touches on the historic accord during a phone call, committing the world’s biggest nuclear powers to deep weapons cuts.

“I’m pleased to announce that after a year of intense negotiations, the United States and Russia have agreed to the most comprehensive arms-control agreement in nearly two decades,” Obama told reporters.

But he could still face an uphill struggle for ratification this year by the U.S. Senate, where support from opposition Republicans will be hard to come by after a bitter fight that ended in congressional approval of his healthcare overhaul.

In Moscow, Medvedev hailed the agreement — which also must be approved by Russian lawmakers — as reflecting a “balance of the interests of both countries.”

Russia made clear, however, that it reserved the right to suspend any strategic arms cuts if it felt threatened by future U.S. deployment of a proposed Europe-based missile defense system that Moscow bitterly opposes.

The agreement replaces a 1991 pact that expired in December. Each side would have seven years after the treaty takes effect to reduce stockpiles of their most dangerous weapons — those already deployed — to 1,550 from the 2,200 now allowed and also cut their numbers of launchers in half.

Secretary of State Hillary Clinton said the new pact sends a message to Iran and North Korea, both locked in nuclear standoffs with the West, of a commitment to thwart nuclear proliferation.

“WE INTEND TO LEAD”

“With this agreement, the United States and Russia — the two largest nuclear powers in the world — also send a clear signal that we intend to lead,” Obama said.

Signaling prospects for cuts by other nuclear powers, British Foreign Secretary David Miliband said: “As soon as it becomes useful to do so, the U.K. stands ready to include our nuclear arsenal in a future multilateral disarmament process.”

German Foreign Minister Guido Westerwelle called it “a milestone that will promote overall nuclear disarmament,” and European Commission President Jose Manuel Barroso congratulated Obama and Medvedev on “this historic agreement.”

The treaty adds another chapter in a quarter century of efforts to make the world safer through nuclear arms control, after a 1986 summit between U.S. President Ronald Reagan and Soviet President Mikhail Gorbachev laid the groundwork.

Obama and Medvedev will sign the new Strategic Arms Reduction Treaty (START) in Prague, capital of the Czech Republic, a former Soviet satellite now in NATO.

The April 8 meeting will be close to the anniversary of Obama’s speech in Prague offering his vision for eventually ridding the world of nuclear weapons, and should help build momentum for a nuclear security summit he will host in Washington on April 12-13.

The treaty does not impose limits on U.S. development of a missile defense system in Europe, which had been a major sticking point in negotiations. Washington insists such an anti-missile shield would be aimed at Iran, not at Russia.

“Missile defense is not constrained by this treaty,” U.S. Defense Secretary Robert Gates said.

But Russian Foreign Minister Sergei Lavrov said either side has the right to stop reducing offensive nuclear weapons if the other beefs up its missile defenses — a warning of consequences if Moscow sees a threat to its security.

“DARKEST DAYS”

Obama said the new treaty would help Washington and Moscow put behind them the “darkest days of the Cold War.”

“It cuts, by about a third, the nuclear weapons that the United States and Russia will deploy,” Obama said. “It significantly reduces missiles and launchers. It puts in place a strong and effective verification regime.

“And it maintains the flexibility that we need to protect and advance our national security, and to guarantee our unwavering commitment to the security of our allies.”

The new pact could strengthen Obama politically, building on the domestic political victory he scored this week when he signed sweeping healthcare reform into law.

Obama still faces a fight to get a two-thirds majority for Senate ratification of the treaty at a time of bipartisan rancor after the bitter fight over healthcare and other parts of his domestic agenda.

Republicans have criticized his national security policies and are in no mood to cooperate, especially ahead of November congressional elections where they hope to score big gains.

Despite that, Clinton insisted the prospects were good for bipartisan support for the treaty.

The final deal also signaled improved relations with Russia that had been badly frayed under Obama’s predecessor George W. Bush. Obama needs Moscow onboard for any further international sanctions against Iran over its nuclear program.

It showed that Moscow and Washington can find a way to work together despite differences over a host of issues from Georgia to missile defense in Europe.

(Additional reporting by Jeff Mason, Ross Colvin, Patricia Zengerle and Susan Cornwell in Washington, Guy Falconbridge and Steve Gutterman in Moscow and Brian Rohan in Berlin; editing by Philip Barbara)