Final answer:
Henry David Thoreau, along with other critics like Abraham Lincoln, suggested that the American government under President Polk was responsible for instigating the Mexican-American War to expand slave territory.
Step-by-step explanation:
Henry David Thoreau suggested that the American government, led by President James K. Polk, was responsible for the Mexican-American War. Thoreau, along with other anti-slavery advocates, believed that the conflict had been deliberately provoked by Polk as part of an expansionist agenda to annex more territory where slavery could be practiced. This view was also reflected in the actions of the Whigs and notable figures like Abraham Lincoln, who criticized the war and demanded transparency from the administration with the “Spot Resolutions.” The war reignited the debate about slavery because it threatened the balance between free and slave states, established by the Missouri Compromise, and raised serious questions about the future of newly acquired territories.