What if the Mexican government had tried American soldiers for "war crimes" during the Mexican-American War?

The Mexican-American War, 1846-48, was the brainchild of President James Polk, and was extremely unpopular both domestically, and internationally. For decades the British government had been accusing the American government of war mongering, with some reason. You can get a feel for their attitude in Charles Dicken's novel "Martin Churzzlewit" (1843). Much of the novel is set in the U.S., and it remains one of the most brilliant satires of American society ever written. Americans are portrayed as ignorant, violent, dishonest, mercenary, war-mongering slave-holders. Essentially, American black people are portrayed as the only fully sane Americans. The Mexican-American War was a straight land grab, and, effectively, Americans could have absorbed the entire empire of Mexico, rather than just the half of it, if they'd wanted. They decided to leave the more populated southern territories of Mexico alone, as more trouble than they were worth. One of the major objections of the U.S. government to Mexico, was that they had outlawed slavery, effectively outlawing the American South's "peculiar institution". As Charles Dickens puts it in "Martin Chuzzlewit", "Americans love Lady Liberty so much, they take liberties with her!" So, let's just suppose, while America is systematically carving up Mexico between 1846 and 1848, the Mexican government decides to capitalize on anti-American sentiment in Great Britain, and hold "war crimes" trials for some captured American soldiers who had assaulted Mexican civilians for one reason or another. What effects would this have had? Presumably, these trials would have received wide attention and newspaper circulation throughout the British Empire. Would the British Empire have provided arms to the Mexican Empire, in response? Would the British Empire have intervened directly in the conflict, with ships and troops? Any thoughts?