During the 1840s, Americans could argue that taking so much of Mexico’s territory constituted extending the area of freedom because of their belief in the idea of Manifest Destiny.
Not all Americans at the time saw the war with Mexico in this way. Many people, particularly free soilers in the North, felt that the war was being waged as a way to extend slavery. They feared that the South was pushing the war as a way to gain more territory that would be converted into slave states.
However, many Americans believed in the ideas of Manifest Destiny. They believed that they were destined to spread their system because they had the best society, the best, government, and the best religion. They believed that taking land from Mexico would spread that superior system over more territory. They would have said that Mexico had a despotic government and a religion (Roman Catholicism) that took away people’s freedoms and made them have to obey their priests, bishops, and the rest of the church hierarchy. For these reasons, they would have said that taking the land and making it American would extend the area of freedom. They would have ignored the issue of slavery because they did not really care about the freedom of African Americans (or any other non-whites) and would instead have focused on the ideas of Manifest Destiny.