I found “A Wicked War” by Amy Greenberg to be well-written and thought provoking. Greenberg weaves together stories of key US individuals while explaining Manifest Destiny and the US politics about slavery, and how these led to the US Civil War.
From Wikipedia, “Manifest destiny was the belief held by Democrats in the United States in the 19th century that American settlers were destined to expand across the continent. … It was used by Democrats in the 1840s to justify the war with Mexico and it was also used to divide half of Oregon with Great Britain.” The US succeeded — this is now part of the US. Americans felt superior to the Native Americans and Mexicans occupying this land, and this helped justify manifest destiny.
This period brought slavery to a head, eventually resulting in the Civil War. Texas declared independence from Mexico in 1936 and then was annexed by the US in 1845, creating a large slave state and temporarily preserving parity between slave states and free states.
A key event is the US-Mexican War, which started in 1846. Greenberg writes about President James Polk, “America’s president invoked a dubious excuse in order to invade a neighboring republic and pursued a war for territory over the objections of a significant portion of Americans. This was unprecedented. All the land taken from Mexico, historians now acknowledge, could have been acquired peacefully through diplomacy and deliberate negotiation.”
The principal military leaders of the Civil War (Robert E. Lee, Ulysses. S. Grant, Stonewall Jackson, and George Meade) gained battle experience while invading Mexico during the US-Mexican War.
Ulysses Grant went on to become the principal Union general in the Civil War and then President of the US. From page 274 of “A Wicked War”, after the war and his presidency,
In 1879 former president Ulysses S. Grant told a journalist “I do not think there was ever a more wicked war than that waged by the United States on Mexico. I thought so at the time, when I was a youngster, only I had not moral courage enough to resign.” At the end of his life he elaborated on his feelings. The U.S.-Mexican War was “one of the most unjust ever waged by a stronger against a weaker nation,” he wrote in his memoirs. The Civil War, he declared, was “our punishment” for that “transgression”.