evidence: According to the treaties between China and Russia, and the subsequent tripartite treaty with autonomous Mongolia, the Province of Uryankhai -- later called Tannu-Tuva -- was considered part of autonomous Mongolia. This province takes in a great basin at the source of the Yenisei River and is surrounded by the mountains of Sayan and Tannu Ola. It is rich in natural resources, both agricultural and mineral. The mountain ranges just named have almost no passes, and the control of this territory by Russia makes the southern borders of Siberia practically impregnable to attack by any possible enemy land forces. Its strategic importance had been recognized by the government of Peter the Great. In those days the Russians had not yet penetrated the Uryankhai Province, and had only a few settlers along the northern parts of Sayan -- the southern reaches of present-day Siberia. Yet in 1715, the Russian Government, in treaties with the Dzungarian Government which laid claim to the Uryankhai Province, asserted: "The Siberian rivers Ob, Yenisei and Lena have been Siberian since time immemorial, from their mouths, where they flow into the sea, to the mountains from which they came.