What significant border issue did Polk resolve with Great Britain?

James K. Polk, during his presidency, addressed the Oregon border issue with Great Britain, which was a significant diplomatic challenge of the time. The Oregon Territory, which was claimed by both the United States and Great Britain due to competing land claims, extended from the Pacific Ocean to the Rocky Mountains and was known for its rich resources and strategic importance.

Polk advocated for the annexation of Texas and sought to expand U.S. territory further into the Pacific Northwest. The slogan "54°40' or Fight!" encapsulated his administration's aggressive stance on claiming the entire Oregon Territory up to the latitude of 54 degrees 40 minutes. However, rather than escalating tensions to war, Polk ultimately negotiated with Great Britain, leading to the Oregon Treaty of 1846. This treaty established the U.S.-Canadian border at the 49th parallel, resolving the issue peacefully and allowing both nations to avoid conflict.

The other options refer to different historical issues. The Missouri Compromise related to the balance of slave and free states, the Louisiana Territory boundary was established through the Louisiana Purchase, and the Mason-Dixon line was a cultural boundary that separated free and slave states. None of these pertain directly to the Oregon border issue, making it the correct answer for