INNER Asia, long asleep within the coccoon of Soviet communism, is stirring again. Mongolia has had a coalition government since democratic elections a year ago. The republics of Soviet Central Asia - Kazakhstan, Uzbekistan, Kirghizia, Tadzhikistan, and Turkmenia - are asserting their independence. Although communist parties are entrenched, opposition voices are beginning to be heard.Beijing, which rules Xinjiang, adjoining the Soviet republics, is concerned. A thousand years ago, it was the Silk Road that linked these lands with China to the east and Persia and Byzantium to the west. From the 1st century B.C. to the days of Marco Polo in the 14th century A.D., the caravan trails that brought Chinese silk to Rome, and Roman gold to China, traversed the great land ocean of the Central Asian steppes and deserts - with two-humped Bactrian camels as ships and Chinese, Central Asian, and Middle Eastern merchants as intrepid navigators. In the Silk Road's heyday, Sogdian merchants, from what is now Uzbekistan, traveled east and west, purveying Chinese silk to the West, and Persian grapes and glass to China. Eventually, invading Arabs wiped out the Sogdian faith and language. Central Asia became a stronghold of Islam, and remained so after the Russian conquest in the 19th century. But with Columbus and the Age of Discovery, Europeans, and later Americans, found the oceans a safer, cheaper, and more comfortable way to trade with distant c ountries, and the Silk Road declined. Throughout the Silk Road's history, waves of conquest swept across it as nomad tribes fought and sometimes vanquished the settled peoples of China, India, Persia, and the Middle East. As China's reach into Central Asia weakened, Russia became the dominant power and eventually the master of these lands. In the wake of the Communist Revolution of 1917, Moscow established five republics in the region - Kazakhstan, Uzbekistan, Turkmenia, Kirghizia, Tadzhikistan. China could not prevent what it called Outer Mongolia from falling under Soviet control as well, but it retained its rule over eastern Turkestan, which it turned first into a province and then, after China's own communist revolution, into the Xinjiang Uighur Autonomous Region. Tibet also became an autonomous region. Unlike Soviet republics, China's autonomous regions do not have even a theoretical right to secede. Beijing sent millions of Chinese to settle in Xinjiang, but Turkic peoples are still the majority, and Beijing is extremely sensitive to what has been going on in the Soviet republics across its border. If these republics establish a new union with Moscow, a union loose enough to satisfy their political aspirations and close enough to assure mutual economic benefits, agitation in Xinjiang for greater freedom from Beijing is bound to grow. So the potential for conflict in the region remains. In the long run, however, the peoples of Inner Asia, from Mongolia to the Caspian Sea, will have to coexist with their two powerful neighbors, China and Russia. And, being landlocked, they must depend on Soviet or Chinese ports in order to participate in international commerce. SOVIET Central Asia is already linked to Europe and the Far East by the Soviet rail system, while Xinjiang's goods reach Shanghai via the Chinese system. The two systems could be linked to each other in Central Asia with relative ease, given the right political conditions. If Iran mellows politically and Afghan turmoil subsides - both admittedly big ifs - outlets to the Persian Gulf and to Pakistan will be possible. As repressive as it was, Soviet rule (and Chinese) introduced Western basics of science and technology, established schools and universities, and laid the foundations of a modern industrial society (along with pollution and environmental damage). The newly emergent peoples of Inner Asia face the challenge of building on this flawed but vital legacy and of evolving a form of democracy that meets their aspirations without falling into the trap of competing nationalisms. To succeed, they will need international help.