Personal Memoirs of U.S. Grant, 1885
Generally the officers of the army were indifferent whether the annexation 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. It extended from the Sabine River on the east to the Rio Grande on the west, and from the Gulf of Mexico on the south and east to the territory of the United States and New Mexico--another Mexican state at that time--on the north and west. 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. Soon they set up an independent government of their own, and war existed, between Texas and Mexico, in name from that time until 1836, when active hostilities very nearly ceased upon the capture of Santa Anna, the Mexican President. Before long, however, the same people--who with permission of Mexico had colonized Texas, and afterwards set up slavery there, and then seceded as soon as they felt strong enough to do so--offered themselves and the State to the United States, and in 1845 their offer was accepted. 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.

Even if the annexation itself could be justified, the manner in which the subsequent war was forced upon Mexico cannot. The fact is, annexationists wanted more territory than they could possibly lay any claim to, as part of the new acquisition. Texas, as an independent State, never had exercised jurisdiction over the territory between the Nueces River and the Rio Grande. Mexico had never recognized the independence of Texas, and maintained that, even if independent, the State had no claim south of the Nueces. I am aware that a treaty, made by the Texans with Santa Anna while he was under duress, ceded all the territory between the Nueces and the Rio Grande--, but he was a prisoner of war when the treaty was made, and his life was in jeopardy. He knew, too, that he deserved execution at the hands of the Texans, if they should ever capture him. The Texans, if they had taken his life, would have only followed the example set by Santa Anna himself a few years before, when he executed the entire garrison of the Alamo and the villagers of Goliad.

In taking military possession of Texas after annexation, the army of occupation, under General Taylor, was directed to occupy the disputed territory. The army did not stop at the Nueces and offer to negotiate for a settlement of the boundary question, but went beyond, apparently in order to force Mexico to initiate war. It is to the credit of the American nation, however, that after conquering Mexico, and while practically holding the country in our possession, so that we could have retained the whole of it, or made any terms we chose, we paid a round sum for the additional territory taken; more than it was worth, or was likely to be, to Mexico. To us it was an empire and of incalculable value; but it might have been obtained by other means. The Southern rebellion was largely the outgrowth of the Mexican war. Nations, like individuals, are punished for their transgressions. We got our punishment in the most sanguinary and expensive war of modern times.