<Li> Northern representatives in the U.S. House of Representatives pass the Wilmot Proviso which would prevent slavery in territory captured from Mexico . Southern Senators block passage of the proviso into law in the U.S. Senate . The Wilmot Proviso never becomes law but it does substantially increase friction between the North and South . Congress also rejects a proposal to extend the Missouri Compromise line to the west coast and other compromise proposals . </Li> <Li> Iowa is admitted to the United States as a free state . </Li> <Tr> <Td> 1847 </Td> <Td> <Ul> <Li> The Massachusetts legislature resolves that the "unconstitutional" Mexican--American War was being waged for "the triple object of extending slavery, of strengthening the slave power, and of obtaining control of the free states". </Li> <Li> John C. Calhoun asserts that slavery is legal in all of the territories, foreshadowing the U.S. Supreme Court's Dred Scott decision in 1857 . </Li> <Li> Democrat Lewis Cass of Michigan proposes letting the people of a territory vote on whether to permit slavery in the territory . This theory of popular sovereignty would be further endorsed and advocated by Democratic Senator Stephen A. Douglas of Illinois in the mid-1850s . </Li> </Ul> </Td> </Tr> <Ul> <Li> The Massachusetts legislature resolves that the "unconstitutional" Mexican--American War was being waged for "the triple object of extending slavery, of strengthening the slave power, and of obtaining control of the free states". </Li> <Li> John C. Calhoun asserts that slavery is legal in all of the territories, foreshadowing the U.S. Supreme Court's Dred Scott decision in 1857 . </Li> <Li> Democrat Lewis Cass of Michigan proposes letting the people of a territory vote on whether to permit slavery in the territory . This theory of popular sovereignty would be further endorsed and advocated by Democratic Senator Stephen A. Douglas of Illinois in the mid-1850s . </Li> </Ul>

How did the nations expansion lead to the civil war