March 15, 1820 – Maine becomes the 23rd U.S. state, after Congress enacts the Missouri Compromise.
From 1604 to 1763, the British, Dutch, and French had fought over Maine, New Hampshire, Newfoundland, Nova Scotia, and Quebec.
To help protect the small and struggling British colonies in the area, King Charles II gave the area between the Kennebec River and the Maine-New Hampshire border to the Massachusetts Bay Colony in 1658. (He reversed this decision in 1676, but Massachusetts regained control by purchasing the area. It later sold these off to private buyers in 1786.) In 1669, Charles II gave the area between the Kennebec and St. Croix rivers to the Province of New York. In 1686, Charles' succesor, James II, reorganized the American colonies. New Jersey, coastal New York, Connecticut, Massachusetts, Rhode Island, southern Vermont, southern New Hampshire, and Maine were merged into a colony known as the Dominion of New England. Ruled directly from England, the provincial government collapsed in 1689 after the "Glorious Revolution" that replaced James II with William and Mary. When the province was re-established in 1691, the Kennebec-St. Croix area was given to the Province of Massachusetts Bay. Northern Maine, however, remained disputed territory.
In 1779, during the American Revolution, the British attempted to turn a part of Maine's northern coast into a colony named "New Ireland" and turn it over to Loyalists. When the British lost the war, they rewarded Loyalists by splitting New Brunswick off from Nova Scotia, and giving it to the Loyalists for resettlement.
During the War of 1812, Maine suffered more than most. Canadian privateers and the Royal Navy effectively shut down shipping in the area, devastating Maine's economy. The U.S. government, generally losing the war badly, assigned only a few forces to Maine, and only in the western areas near New Hampshite. Massachusetts declined to protect her own territory, focusing all its efforts on Boston and the coast. The British, fighting Napoleon in Europe, ended the war by restoring the "status quo ante" and returning Maine to the United States.
Maine's vulnerability to foreign invasion, and its lack of protection by Massachusetts, led the people of Maine to seek statehood. Between the end of the war in 1815 and 1915, Maine essentially developed into a near-autonomous state. It elected its own legislature and governor, passed laws, raised a militia, and did everything but declare its independence. Massachusetts didn't do a thing to stop it.
In the spring of 1819, voters in Maine established a constitutional convention to seek statehood. The U.S. Constitution barred the creation of a state from an existing state, unless the existing state gave its consent. Massachusetts willingly did so. The Maine constitutional convention issued a draft state constitution in October 1819.
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Meanwhile......................... slave-holding Louisiana had become a state in 1812. The remainder of the immense Lousiana Purchase (now known as the Missouri Territory) had seen a huge influx of settlers, primarily from the slave-holding South since the end of the war. The territorial governor had organized the heavily-settled area now known as Missouri into five counties in 1812, and these settlers were predictably opposed to any meddling with their slave-holding rights. By 1818, the "Missouri counties" near nearing the population threshhold at which the area could become a state.
As Congress debated Missouri statehood in 1819, Rep. James Talmadge (R-N.Y.) proposed an amendment barring slavery in Missouri. Talmadge was personally opposed to slavery, and he was supported by Rep. John W. Taylor (R-N.Y.), who had almost barred slavery in Arkansas Territory earlier in the year.
The Talmadge amendment ruptured the Jeffersonian Republicans, the only political party that existed in Congress as the time. Since 1787, American politics had acknowledged state sovereignty when it came to slavery. In exchange, slave-holding states supported the abolition of slavery in the Midwest and the outlawing the trans-Atlantic slave trade. But the South, which had more political power due to the three-fifths compromise in the Constitution which counted slaves as population, was now losing this power due to the rapidly growing Northern population. Now the Northern Jeffersonian Republicans embraced the Declaration of Independence's call for equality of all people, while Southern Jeffsonian Republicans rejected the Declaration in favor of the Constitution and their own interpretation of it. As the South lost voting power in the House, it retained it in the Senate (where the three-fifths compromise did not matter).
Although the Talmadge amendment died in the Senate, the debate did not. Southern Jeffsonian Republicans were faced with the prospect of stalemate, in which no new states (slave or free) could be organized. Slavery would be confined to the South. At the same time, Southern Jeffsonian Republicans would be forced into denying fundamental principles of democracy and representation which they strongly supported.
Finally, the Southern Jeffsonian Republicans were deeply concerned that the Northern Jeffsonian Republicans might split off to form a Free Soil Party. This would permanently destroy any chances of using party solidarity to force the admission of Missouri as a slave state.
A compromise was reached: Maine would be admitted as a free state, and Missouri as a slave state. This would keep the status quo in the Senate (winning the votes of Northern Jeffsonian Republicans), while allowing free-state Representatives to abandon the majority in the House and support admission of Missouri.
Another part of the compromise included the barring of slavery in the Missouri Territory north of 36°30′ north. This effectively kept slavery Southern (and would admit Texas as a slave-state later), appeared to end the debate over slavery.
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The Missouri Compromise was enacted on March 4, 1820.
Maine gained its statehood on March 15, 1820. The state's northern boundary would not be established until ratification by the United States and United Kingdom of the Webster-Ashburton Treaty in 1839.
Missouri gained its statehood on August 10, 1821.
The rupture in the Jeffersonian Republicans would destroy the party in 1828, leading to the creation of the Democratic Party and the Whig Party.
Thomas Jefferson, a frail 76-year-old in 1820, called the Missouri Compromise the "death knell" of the Union. Once a free-slave line was established, he felt, any attempt to violate the line would merely aggravate the passions of both sides. Over time, the line would become more deeply embedded in American politics, worsening the outrage people felt if it was touched. Eventually, the Union would come asunder.
Although intended to allow the swift admission of new states to the Union, no state was admitted until Arkansas in June 1836. It was balanced by Michigan in January 1837. The political balance was briefly upended with the admission of slave-holding Florida in March and slave-holding Texas in December 1845. But the balance was restored by the admission of free-state Iowa in December 1846 and free-state Wisconsin in May 1848.
With Northern populations rising faster than Southern, the Missouri Compromise fell apart with the admission of California in September 1850. Senator Stephen Douglas (R-Illinois) attempted to save it by proposing the Kansas-Nebraska Act in 1854. This legislation repealed the Missouri Compromise, and organized the Kansas and Nebraska territories. Admission of these rapidly-growing territories as slave or free would be based on popular sovereignty. It also called for the construction of a transcontinental railroad along either a northern, southern or middle route. Southerners loved the bill, as they believed only the southern route was feasible. This would encourage massive settlement along the railroad line, rebuilding white populations in the slave-holding South.
In fact, the Kansas-Nebraska Act caused extensive violence as free-state and slave-state whites fought for dominance in the new territories. It precipitated the Civil War six years later.
Oddly, Washington, Idaho, and Montana owe their existence to the Kansas-Nebraska Act. Surveys of the areas were conducted under the auspices of the act, leading to the first American surveys since Lewis & Clark in 1805. Identification of a number of passes over the Rocky and Bitterroot mountains, exploration of the Bitterroot and Flathead valleys, and construction of the essential Mullan Road all came about because of the Kansas-Nebraska Act. Surveyor John Mullan came up with the name "Montana", and his participation in territorial politics led to the drawing of the current boundaries of these three states.