Why did Polk first demanded all of the Oregon Territory from Britain and then agree to compromise?

Answer 1

Polk did not really want to fight Britain. Instead, in 1846, he agreed to a compromise. Oregon was divided at latitude 49°N. Britain got the lands north of the line, and the United States got the lands south of the line.

Answer 2

War was also threatening with Mexico, so he agreed to a reasonable compromise with Britain over Oregon and pushed for war with Mexico. He also probably preferred a war with Mexico to a war with Britain.

Answer 3

Much of Oregon was already a territory of the United States. The problem was the location of the northern border between Oregon and the British territory of Canada and its exact location was a campaign issue in 1844.