Russia and the Soviet Union were home to more ethnic groups than any other country in the world, the result of centuries of expansion and conquest. The regimes between 1855 and 1991 adopted a variety of approaches to integrating them into an overall national entity, sometimes through concessions, sometimes through coercion, but most frequently through a combination of the two. The main contrast, however, was between the unitary system of the Tsars and the federalism adopted as the official policy of the Communist regime.