The Lost Cause mythology began taking hold immediately after the Civil War. Here's Grant comparing how Confederates were choosing to remember the fight with how Mexicans remembering their war with America. For context, it's worth noting that Grant was opposed to the Mexican-American War, though he did served in it:
The Mexican army of that day was hardly an organization. The private soldier was picked up from the lower class of the inhabitants when wanted; his consent was not asked; he was poorly clothed, worse fed, and seldom paid. He was turned adrift when no longer wanted. The officers of the lower grades were but little superior to the men. With all this I have seen as brave stands made by some of these men as I have ever seen made by soldiers. Now Mexico has a standing army larger than that of the United States. They have a military school modelled after West Point. Their officers are educated and, no doubt, generally brave. The Mexican war of 1846-8 would be an impossibility in this generation.
The Mexicans have shown a patriotism which it would be well if we would imitate in part, but with more regard to truth. They celebrate the anniversaries of Chapultepec and Molino del Rey as of very great victories. The anniversaries are recognized as national holidays. At these two battles, while the United States troops were victorious, it was at very great sacrifice of life compared with what the Mexicans suffered. The Mexicans, as on many other occasions, stood up as well as any troops ever did. The trouble seemed to be the lack of experience among the officers, which led them after a certain time to simply quit, without being particularly whipped, but because they had fought enough.Their authorities of the present day grow enthusiastic over their theme when telling of these victories, and speak with pride of the large sum of money they forced us to pay in the end. With us, now twenty years after the close of the most stupendous war ever known, we have writers--who profess devotion to the nation--engaged in trying to prove that the Union forces were not victorious; practically, they say, we were slashed around from Donelson to Vicksburg and to Chattanooga; and in the East from Gettysburg to Appomattox, when the physical rebellion gave out from sheer exhaustion. There is no difference in the amount of romance in the two stories.
I would not have the anniversaries of our victories celebrated, nor those of our defeats made fast days and spent in humiliation and prayer; but I would like to see truthful history written. Such history will do full credit to the courage, endurance and soldierly ability of the American citizen, no matter what section of the country he hailed from, or in what ranks he fought. The justice of the cause which in the end prevailed, will, I doubt not, come to be acknowledged by every citizen of the land, in time. For the present, and so long as there are living witnesses of the great war of sections, there will be people who will not be consoled for the loss of a cause which they believed to be holy. As time passes, people, even of the South, will begin to wonder how it was possible that their ancestors ever fought for or justified institutions which acknowledged the right of property in man.
Bad-ass as always. Of course the way they get around this is by claiming the never stood for "the of property in man."