Complex sentence: Mountrail County was first established in 1873 (as Mountraille County) by the Dakota Territorial Legislature, but the boundaries were different from what they are today, extending from the Missouri River north to the border with Canada.
Split into simpler sentences: Mountrail County was first established in 1873 (as Mountraille County) by the Dakota Territorial Legislature. The boundaries extended from the Missouri River north to the border with Canada.