The passage of the 1854 Kansas–Nebraska Act and the outbreak of violent confrontations between pro-slavery and anti-slavery forces in Kansas exacerbated sectional tensions and split both major parties. Pierce was denied re-nomination in favor of James Buchanan, while the Whig Party collapsed. In the 1856 presidential election, Buchanan defeated John C. Frémont of the anti-slavery Republican Party and former President Fillmore, the candidate of the nativist American Party. Sectional tensions continued to escalate after the Supreme Court handed down its decision in "Dred Scott v. Sandford". Buchanan proved incapable of healing sectional divides, and some leading Southerners became increasingly vocal in their desire to secede from the union. In 1859, Buchanan assigned Scott to lead a mission to settle a dispute with Britain over the ownership of the San Juan Islands in the Pacific Northwest. Scott reached an agreement with British official James Douglas to reduce military forces on the islands, thereby resolving the so-called "Pig War".