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.

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.

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.