Tajiks trace their heritage back to ancient Persian-speakers who arrived in modern-day Central Asia some thousands of years ago; some groups in Tajikistan speak languages that have changed little from those remarked upon by Alexander the Great's armies as they passed through the area. The country of Tajikistan, a mountainous series of high plateaus and plunging valleys, was carved out of the Uzbek Soviet Socialist Republic in its current form in 1929, although the people of these two republics have remained closely interlinked linguistically and culturally. Following its declaration of independence and the fall of the USSR in 1991, a struggle for political dominance grew into a civil war lasting from 1992 to 1997. Although hostilities were officially ended in 1997 and members of the opposition subsequently included in the Tajik government, tensions remain in some regions of Tajikistan between previous military opponents. One of the poorest nations in the former USSR, Tajikistan has striven since the end of its civil war to reestablish economic stability and rebuild its export sector.