The period of the war and of the years immediately succeeding witnessed a very rapid growth of population, and a notable swelling of the tide of emigration westward. In 1816 Indiana came within the circle of States; followed alternately by slave and free states--Mississippi, 1817; Illinois, 1818; Alabama, 1819; Maine, 1820. and Missouri, 1821. The great highway built between Cumberland and Wheeling was all alive in those days with the wagons and groups of new settlers. A long era of peace was to follow, and to give the country opportunity to increase, to develop its resources, and to make rapid progress in its prosperity and the development of its institutions. XI. THE MEXICAN WAR. [Sidenote: An Era of Peace.] [Sidenote: Andrew Jackson.] An interval of over thirty years elapsed between our second war with Great Britain and the war with Mexico. Although this period was one of external, and, excepting the troubles which now and then arose with the Indians, of internal peace, its social and political aspects are very full of interest. Within its limits the first railway and the first telegraph-lines were laid in the United States, and the great Erie Canal was built. After three tranquil presidential terms, presided over by the sensible though not brilliant Monroe, and by the shrewd, scholarly, and positive younger Adams, a man succeeded to the Executive Chair whose course was destined to revolutionize parties, to carry party bitterness to a height of great violence, and to divert the political destinies of the country into new channels. Andrew Jackson was well fitted by his strong will and stubborn courage to do the dangerous work of his time. [Sidenote: Nullification.] Various considerations induced the State of South Carolina to defy the Union. The alleged ground of her quarrel was the high rates of the tariff imposed by Congress upon imports. This tariff she resolved to resist; hence a resolution was passed by a convention in South Carolina that after a certain date the tariff should be null and void within her limits. It was further resolved that if the United States attempted to enforce it, South Carolina should secede, and form an independent government. John C. Calhoun was, or was charged with being, the instigator of this movement. It was at once quelled, however, by the prompt action of President Jackson. He sent troops and war-ships to Charleston, under the command of General Scott; and "nullification" was overawed and defeated.