of state) proclaimed that there was an "irrepressible conflict" between slavery and free labor, and that slavery had left the South backward and undeveloped. As the Whig party dissolved in the 1850s, the mantle of abolition fell to its newly formed successor, the Republican Party. Territorial crisis Manifest destiny heightened the conflict over slavery. Each new territory acquired had to face the thorny question of whether to allow or disallow the "peculiar institution". Between 1803 and 1854, the United States achieved a vast expansion of territory through purchase, negotiation, and conquest. At first, the new states carved out of these territories entering the union were apportioned equally between slave and free states. Pro- and anti-slavery forces collided over the territories west of the Mississippi. The Mexican–American War and its aftermath was a key territorial event in the leadup to the war. As the Treaty of Guadalupe Hidalgo finalized the conquest of northern Mexico west to California in 1848, slaveholding interests looked forward to expanding into these lands and perhaps Cuba and Central America as well. Prophetically, Ralph Waldo Emerson wrote that "Mexico will poison us", referring to the ensuing divisions around whether the newly conquered lands would end up slave or free. Northern free-soil interests vigorously sought to curtail any further expansion of slave territory. The Compromise of 1850 over California balanced a free-soil state with a stronger federal fugitive slave law for a political settlement after four years of strife in the 1840s. But the states admitted following California were all free: Minnesota (1858), Oregon (1859), and Kansas (1861). In the Southern states, the question of the territorial expansion of slavery westward again became explosive. Both the South and the North drew the same conclusion: "The power to decide the question of slavery for the territories was the power to determine the future of slavery itself." By 1860, four doctrines had emerged to answer the question of federal control in the territories, and they all claimed they were sanctioned by the Constitution, implicitly or explicitly. The first of these theories, represented by the Constitutional Union Party, argued that the Missouri Compromise apportionment of territory north for free soil and south for slavery should become a constitutional mandate. The failed Crittenden Compromise of 1860 was an expression of this view. The second doctrine of congressional preeminence was championed by Abraham Lincoln and the Republican Party. It insisted that the Constitution