Southerners, outraged at these northern attempts to eliminate slavery, blocked the Tallmadge Amendment in the Senate. Congress was deadlocked over the issue for months until Henry Clay orchestrated the Missouri Compromise. In exchange for admitting Missouri as an unrestricted slave state, Southerners agreed to admit Maine as a free state, thus preserving the sectional balance. The compromise also declared slavery illegal north of the 36° 30' parallel west of Missouri.
In hindsight, the Missouri crisis reveals that the sectional differences that led to the Civil War were present decades before open conflict finally broke out in the 1860s. Antislavery northerners had wanted to end the “peculiar institution” of slavery as far back as the Constitutional Convention. Southerners, however, had become completely dependent on slave labor to produce “king cotton.” Southern elites reasoned that if slavery didn’t expand westward, the southern way of life would certainly die.
During Monroe’s term, the Supreme Court, still under diehard Federalist Chief Justice John Marshall, issued a series of landmark rulings that also increased the power of the federal government:
McCulloch v. Maryland was perhaps the most influential of Marshall’s rulings because it legitimized Hamilton’s belief that the Constitution had been “loosely constructed” to allow the federal government to act in the best interests of the people. Marshall’s decision thus gave future presidents and congresses a green light to enact a wide variety of legislation in accordance with the “spirit” of the Constitution. Likewise, Marshall’s Cohens v. Virginia ruling was highly influential because it helped establish the Supreme Court as the highest law of the land.
Monroe and his administration did much to cement a formal U.S. foreign policy. Secretary of State John Quincy Adams negotiated the Treaty of 1818with Britain that set the border with Canada from Minnesota to the Rocky Mountains at the 49th parallel. The treaty also specified that the United States would occupy the Oregon Territory (present-day Oregon, Idaho, Washington, British Columbia, and part of Montana) jointly with Britain until 1828.
In the face of a united Britain and United States, Russia abandoned its claims to Oregon in the Russo-American Treaty of 1824. Meanwhile, General Andrew Jackson illegally seized Florida from Spain, on the pretext that Spain was plotting with the Seminole tribe against the United States. Spain ceded Florida to the United States in 1819 in exchange for Washington’s retraction of its claims to Texas.