Complex sentence: He was the first commander of the Department of Alaska, from March 18,1868, to June 1,1870, during which he commanded the Russian population of Sitka, Alaska, to abandon their homes as he maintained that they were needed for Americans.
Split into simpler sentences: He was the first commander of the Department of Alaska, from March 18,1868, to June 1,1870. During this time, he ordered Russian residents of Sitka, Alaska to leave their homes, as he maintained that they were needed for Americans.