By Alexander TanasCHISINAU (Reuters) - President Vladimir Putin revoked on Tuesday a 2012 decree that in part underpinned Moldova's sovereignty in resolving the future of the Transdniestria region - a Moscow-backed separatist region which borders Ukraine and where Russia keeps troops. The Russian-speakers of Transdniestria seceded from Moldova in 1990, one year before the dissolution of the Soviet Union, amid fears that Moldova would merge with Romania, whose language and culture it broadly shares. A brief war pitted newly independent Moldova against the separatists in 1992