Complex sentence: Although the letters were anonymous, their source is known; both were written by Anthony Butler, at the time the American minister to Mexico, who was attempting to purchase Texas and felt that rebellion might persuade Mexico to sell.
Split into simpler sentences: Although the letters were anonymous, their source is known; both were written by Anthony Butler, at the time the American minister to Mexico. Butler was attempting to effect the purchase of Texas by the United States, and felt that rebellion might persuade Mexico to sell.