The U.S. Congress was reluctant to declare war on Mexico for several reasons. One of the primary reasons was the concern over the expansion of slavery. At the time, the United States was deeply divided over the issue of slavery, and many members of Congress were concerned that the acquisition of new territories would tip the balance of power between free and slave states. Additionally, some members of Congress were skeptical about the legality and morality of going to war to acquire territory. Others believed that they could achieve their territorial goals through diplomacy rather than war. Finally, some members of Congress were concerned about the potential cost of a war with Mexico and the impact it could have on the U.S. economy.
So, option 2 since it’s the primary reason.