The U.S. - British boundary dispute ("Fifty-Four Forty or Fight")
In 1844 presidential candidate James K. Polk's primary campaign issue was to expand the United States to include Texas and the Pacific Northwest. Polk's battle cry was "Fifty-four forty or fight," which meant the United States would accept nothing less from the British than all of the Oregon Country, as far north as the border of Alaska.
Polk won the Presidency and took office in 1845. However, his victory was by a very small margin, and he did not feel a mandate was given to declare war on Britain over the Pacific Northwest. On the other hand, the British were incensed by Polk's rhetoric and made preparations for war. In 1845 two British spies, Lieutenant Warre and Lieutenant Vavasour, arrived on the Columbia River to survey the area and provide intelligence to the British in case of war.
Eventually both sides realized Oregon was not worth a war. After some tense negotiations, on August 5, 1846, the present boundary along the 49th Parallel was decided as the border between the British and American lands, giving the United States clear title to present-day Idaho, Oregon and Washington.