Complex sentence: On February 24,1863, during the American Civil War, Congress established the new Arizona Territory by splitting off the western portion of the New Mexico Territory-lands that would ultimately become the state of Arizona and the southernmost part of Nevada.
Split into simpler sentences: On February 24,1863, during the American Civil War, Congress established the new Arizona Territory by splitting off the western portion of the New Mexico Territory. Eventually this area was organized as the state of Arizona and the southernmost part of Nevada.