US President Barack Obama and his Russian counterpart, Dmitry Medvedev are signing an historical nuclear treaty in Prague today. The START II nuclear weapons reduction treaty was agreed by both leaders to limit nuclear warheads to 1,550 warheads, about 30 per cent less than currently allowed.. The deal replaces the 1991 Strategic Arms Reduction Treaty which expired last December. Today's treaty signing is being seen as an important step toward an improvement in relations between Moscow and Washington. Relations went into deep freeze in 2008, following the war in Georgia. And it is an especially important first step for President Obama's drive to reduce nuclear arsenals worldwide.