“I more than suspect already that he is deeply conscious of being in the wrong — that he feels the blood of this war, like the blood of Abel, is crying to Heaven against him.
“That originally having some strong motive — what, I will not stop now to give my opinion concerning — to involve the two countries in a war, and trusting to escape scrutiny by fixing the public gaze upon the exceeding brightness of military glory… he plunged into it and has swept on and on till, disappointed in his calculation of the ease with which Mexico might be subdued, he now finds himself he knows not where.
“How like the half insane mumbling of a fever-dream is the whole war part of his late message! His mind, taxed beyond its power, is running hither and thither, like some tortured creature on a burning surface, finding no position on which it can settle down and be at ease.”
That’s Abraham Lincoln’s opinion of James Polk during the Mexican War.
The “strong motive” on which Lincoln did not give his opinion at the time, was the desire to create more slave states. Slavery had to expand to survive. That’s why there was such a political battle about slavery in the territories.
More interesting stuff from Gary Wills book, Lincoln at Gettysburg.