Answer:
Polk accomplished this through the annexation of Texas in 1845, the negotiation of the Oregon Treaty with Great Britain in 1846, and the conclusion of the Mexican-American War in 1848, which ended with the signing and ratification of the Treaty of Guadalupe-Hidalgo in 1848.
The United States Constitution does not specifically address the secession of states and the issue was a topic of debate from after the American Revolutionary War and until the Civil War, when the Supreme Court ruled, in Texas v. White, that states cannot unilaterally secede, except through revolution or through the expressed consent of the other States.
