1. The United States was justified in going to war because Mexico had shed American blood on American soil, Texas (a land that many Mexicans still considered theirs).
2.
Northerners opposed annexation because they feared that annexation of more slave territory would tip the uneasy balance of the Senate in favor of slave states---and prompt war with Mexico.
3. Texans wanted independence from Mexico because of Mexico's abolition of slavery, increase in tariffs, and the rise of Santa Anna. Texas, being Mexican territory, was subject to Mexican laws and policies. In 1831, Mexico abolished slavery.
4. They hoped to prevent border violations by horse thieves and to protect the territory from Native American attacks.