Ulysses S. Grant's reflections on the war
Generally the officers of the army were indifferent whether the annexation [of Texas] was consummated or not; but not so all of them. For myself, I was bitterly opposed to the measure, and to this day regard the war, which resulted, as one of the most unjust ever waged by a stronger against a weaker nation. It was an instance of a republic following the bad example of European monarchies, in not considering justice in their desire to acquire additional territory. Texas was originally a state belonging to the republic of Mexico. . . . An empire in territory, it had but a very sparse population, until settled by Americans who had received authority from Mexico to colonize. These colonists paid very little attention to the supreme government, and introduced slavery into the state almost from the start, though the constitution of Mexico did not, nor does it now, sanction that institution. . . . The occupation, separation and annexation were, from the inception of the movement to its final consummation, a conspiracy to acquire territory out of which slave states might be formed for the American Union.
Personal Memoirs of U.S. Grant, 1885