Why were northern states against the annexation of Texas?

Following the 16th century conquests of Hernando Cortez, Spanish forces extended their influence northward into what is the present-day United States. This expansionestablished a Spanish (and later Mexican) claim to much of the southwestern part of the United States and California.

Increasing numbers of American settlers filled the region north of the Rio Grande, particularly in the 1820s and 1830s. This resulted in a successful drive for Texas Independence and a push for annexation that soon after the United States recognized the Texas Republic on March 3, 1837.