Russia's expansion to the south and subsequent need to fortify and protect its borders in the Transcaucasus has been a primary feature of Russian policy for two centuries. That Russian insecurity about its borders intensified during the twentieth century is proven by the Soviet-Iranian Treaty of 1921 which subsequently permitted the Soviet Union to extend even farther: a paragraph of this treaty permitted it to occupy northern Iran in 1941, in order to "take the necessary measures in the interests of self-defence". The collapse of the USSR, however, forced Russia to withdraw into the North Caucasus which constituted the new southern border of the Russian Federation. At the same time, it was clear to Russian policy makers that the allegiances of the newly independent states of Azerbaijan, Georgia and Armenia were no longer necessarily with Moscow, but were more westward-looking. This required that a new border policy be generated both with regard to the North Caucasus and to ensure that the interests of the countries of the Transcaucasus would not do damage to Russian interests. The result can be described as a policy seeking to maintain an imbalance of power.