Complex sentence: The U.S. Senate ratified the treaty by a vote of 38--14, against the Whigs who had opposed the war, rejected Manifest Destiny in general and rejected this expansion in particular.
Split into simpler sentences: The U.S. Senate ratified the treaty by a vote of 38--14. The opponents of this treaty were led by the Whigs, who had opposed the war and rejected Manifest Destiny in general, and rejected this expansion in particular.