Justification refers to providing valid reasons or evidence to support a decision or action. In the context of President Polk's War Message to Congress, it means presenting the reasons for going to war with Mexico.
Related terms
Manifest Destiny: The belief that it was America's destiny and right to expand its territory from coast to coast.
Annexation: The act of incorporating new territory into an existing country.
Territorial disputes: Conflicts between nations over control and ownership of land and borders.