New Delhi, Feb. 5 -- The last remaining nuclear arms control treaty between the United States and Russia expires today, removing caps on the world's two largest nuclear arsenals for the first time in decades and heightening fears of a new, unconstrained arms race.

The Strategic Arms Reduction Treaty - known as New START - was signed in 2010 and has been regarded as a cornerstone of strategic stability, limiting the number of deployed strategic nuclear warheads and providing transparency measures designed to prevent miscalculation between Washington DC and Moscow.

New START capped the number of deployed strategic nuclear warheads for each side at 1,550. It also limited each party to no more than 700 deployed missiles and bombers, and est...