(Reuters) – The U.S. and Russian presidents are to sign a pact on Thursday committing the former Cold War foes to unprecedented nuclear arms reductions, cementing a hard-won deal that should put strained ties on firmer footing.

World | Barack Obama | Russia

After nearly a year of tough negotiations, the signing by Barack Obama and Dmitry Medvedev in Prague, the capital of a former Soviet satellite now in NATO, will symbolize cooperation between Washington and Moscow for the sake of global security.

Both presidents say new cuts in the largest arsenals on the planet are a step toward a world without nuclear weapons and a signal to nations seeking them that there is no need.

But the successor to the 1991 Strategic Arms Reduction Treaty (START I) will not come into force without ratification by lawmakers in both countries, and could face a rough ride in the U.S. Senate.

Analysts say it will be no cure-all for Russian-American relations, which have improved after hitting a post-Soviet low during Russia’s 2008 war with Georgia but remain troubled by a range of disputes.

Neither will the START successor deal resolve simmering tension over missile defense, which has haunted ties since the Reagan era and hurt them badly in the past decade.

With Russia saying it could withdraw from the pact if its security is threatened by U.S. missile defences, the divisive issue could come to the fore again.

Russia has long complained that cutting its offensive arsenal could leave it exposed if the United States builds a missile shield in Eastern Europe. Prime Minister Vladimir Putin clouded hopes for the offensive weapons pact by suggesting in December that it should also limit missile defences.

The pact is expected to acknowledge a link between offensive and defensive weapons, but U.S. officials have stressed it will not restrict the development of missile defences.

The United States says its defensive plans are no threat to Russia.

The Kremlin’s top foreign policy adviser, Sergei Prikhodko, said on Friday that Russia would underscore its right to bow out of the pact in response to U.S. missile defences in a unilateral declaration alongside the treaty, Russian news agencies reported.

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Moscow’s concerns about missile defense and Western conventional weapons superiority also mean further nuclear arms agreements Obama hopes can follow will be far harder to secure.

“It took 10 months, but this treaty is going to be fairly easy compared to the next one,” said Steven Pifer, a senior fellow at the Brookings Institution.

Negotiators missed an initial target of December 5, when START I expired, and failure still seemed possible until both sides announced late last month that Obama and Medvedev would meet in Prague on April 8 to sign the pact.

The treaty would limit the number of operationally deployed nuclear warheads to 1,550 for each country — down nearly two-thirds from START I and 30 percent lower than the ceiling of the 2002 Moscow Treaty set for each side by 2012.

The signing will be the first major concrete foreign policy achievement for Obama, who has sought to “reset” Russia ties.

It will pave the way for a nuclear security summit he is hosting the following week — hoping to marshal broader support in standoffs with Iran and North Korea — and a May conference meant to bolster the global nuclear Non-Proliferation Treaty.

But there is no guarantee it will translate into stronger Russian backing for U.S. policy on Iran and Afghanistan.

“Agreeing a strategic arms treaty is a big achievement, but it does not automatically carry over into other aspects of relations,” said Fyodor Lukyanov, editor of the journal Russia in Global Affairs.

Signing a major nuclear weapons treaty is also a boost for Medvedev, still in Putin’s shadow. The pact with the United States will remind the world of Russia’s nuclear might.

And it is in the Kremlin’s interest because Russia’s aging arsenal would likely drop to the limits set by the treaty in several years anyway, analysts say.

But the signing will leave one crucial hurdle: ratification.

Even without limits on missile defense, which would have ruined the treaty’s chances in the Senate, Obama may have trouble securing the 67 votes needed — particularly if the process drags on beyond November elections in which his Republican foes are expected to pick up seats.

The Kremlin faces no such challenge from Russia’s docile parliament. But Moscow has urged “synchronized” ratification, hinting it will hold back until Senate support is assured.

“Russia is ready to ratify the treaty, but Russia absolutely does not want to find itself in a position where it ratifies and the Senate does not,” Lukyanov said. (Additional reporting by Matt Spetalnick; editing by Myra MacDonald)