I do think this is a fascinating case of a soldier admitting to a moral (or ethical?) fault. However, as I said in a previous post, I wonder whether Grant introduces this dilemma with a rhetorical purpose in mind. His memoirs were written 20 years after the end of the Civil War, so he had time to think about the narrative he might want to create.
I think he might be starting up his discussion of the Mexican War so as to connect it with the Civil War and the slavery issue. If you read the context in pages 32-34, you might see what I'm talking about. Here are some excerpts:
"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...
"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."
("Personal Memoirs of U.S. Grant." NY: C.L. Webster & Co., 1885-86. Pages 32-34)
I'm not saying there was anything nefarious in the way Grant structured his narrative, but I do wonder whether his admission to a lack of moral courage in Mexico was in part meant as a way to draw the reader in to his larger argument about slaveholders' efforts to enlarge their territory.
As I said before, I'm sure some of our colleagues here have read Grant's memoirs in their entirety, whereas I haven't. Somebody else might have more enlightening comments than my poor effort!
ARB