Complex sentence: Most of Assiniboia was made part of the U.S.A. in 1812 and was transferred back in 1869, in 1870 the Riel Rebellion would effectively mean the area was under Metis control from 1870.
Split into simpler sentences: Most of Assiniboia was made part of the US in 1812 and was transferred back in 1869. In 1870 the Riel Rebellion would effectively mean the area was under Metis control from 1870.