The opposition of Northerners to the Mexican-American War stemmed from a combination of moral, political, and social reasons. Many Northerners viewed the war as an unjust aggression against Mexico, seeing it as an attempt to expand slave territory.
One major factor was the fear that the acquisition of new territories would lead to the expansion of slavery, which many Northerners opposed on moral grounds. The debate over whether new states admitted to the Union would be free or slave states heightened tensions between the North and the South. This concern was articulated by prominent figures such as Congressman David Wilmot, who introduced the Wilmot Proviso, aiming to ban slavery in any territory acquired from Mexico.
Moreover, there was a perception among many Northerners that the war was being waged for the interests of Southern slaveholders, who would benefit from increased land for cotton production. This perception led to accusations that the war was a ‘slaveholders’ war,’ further galvanizing Northern resistance.
Lastly, there were also staunch abolitionists and pacifists who condemned the war on ethical grounds, arguing that it was wrong to engage in military conflict for the purpose of acquiring land and resources. This moral opposition contributed to the overall anti-war sentiment in the North.