As an avowed expansionist, Pierce sympathized with the filibuster government set up in Nicaragua by William Walker, and finally accorded it recognition.
An enthusiastic believer in the destiny of his country and more especially of the West, and a thoroughgoing expansionist, he heartily favoured in Congress the measures which resulted in the annexation of Texas and in the Mexican War - in the discussion of the annexation of Texas he suggested as early as 1845 that the states to be admitted should come in slave or free, as their people should vote when they applied to Congress for admission, thus foreshadowing his doctrine of " Popular Sovereignty."
Polk was an ardent expansionist, but the old idea that his policy was determined entirely by a desire to advance the interests of slavery is no longer accepted.