The Mexican War and its aftermath

General Winfield Scott enters Mexico City
13 September 1847
Public domain

‘Manifest Destiny’

Andrew Jackson's recognition of the Lone Star Republic exacerbated the already existing tensions over slavery. For the next seven years there was continuous agitation to bring Texas into the Union. Though many American politicians correctly predicted that the annexation of Texas would bring problems,  the tide was flowing against their caution. This was the period of ‘manifest destiny'. To people like the Democratic newspaper editor John O’Sullivan, it was ‘manifest destiny’ that the United States would soon possess not only Texas but Oregon and later California. 

The flag of the Republic of Texas
the 'Lone Star Republic',
officially adopted 1839

The annexation of Texas

Even before O’Sullivan had coined the term, the issue of manifest destiny was a live one in politics. In the run-up to the election of 1844 the former President, Martin van Buren, opposed the annexation of Texas because he wanted to stop the expansion of slavery.  He was backed by a majority in Congress. Henry Clay spoke for many when he asserted that ‘annexation and war with Mexico were identical’. But as a result, Van Buren lost the Democratic nomination to to James K. Polk, the former Governor of Tennessee, who had gained the support of Andrew Jackson. Polk’s win was a victory for manifest destiny. 


Polk was a fire-breather, but he chose his opponents carefully. In

June 1846 he was to sign the Oregon Treaty with the British that settled the boundary between Oregon and British Columbia at the 49th parallel, with the exception of Vancouver Island, which remained British. But while Britain was powerful, Mexico was not.

On 26 February 1845, six days before Polk took office, Congress approved the annexation of Mexico. This came into effect on 29 December when Polk signed legislation making Texas the 28th state of the Union. It was the decisive step making war with Mexico inevitable.

The Mexican-American War

On 23 April 1846 the President of Mexico issued a proclamation declaring Mexico’s intention to fight a defensive war against the United States. On April 25 a 2,000-strong Mexican cavalry detachment attacked a seventy-man U.S. patrol under the command of Captain Seth Thornton, which had been sent into the contested territory north of the Rio Grande and south of the Nueces River, killing sixteen American soldiers.  On May 11, Polk stated to Congress stated that 

‘Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon American soil’. 

Congress approved the declaration of war on May 13, with southern Democrats in strong support. But sixty-seven Whigs voted against the war, a sign of rising opposition, particularly in the North, where people assumed that Polk wanted the war in order to acquire more slave territory.   Among the minority voices was that of the former president, John Quincy Adams, who called it ‘a most unrighteous war’. 

The Wilmot Proviso: The war was to last two years. In August 1846 Davis Wilmot, a Democratic Congressman from Pennsylvania introduced an amendment to a bill authorising the granting of money in order to facilitate negotiations with Mexico, known as the Wilmot Proviso.

Provided, That, as an express and fundamental condition to the acquisition of any territory from the Republic of Mexico by the United States, by virtue of any treaty which may be negotiated between them, and to the use by the Executive of the moneys herein appropriated, neither slavery nor involuntary servitude shall ever exist in any part of said territory, except for crime, whereof the party shall first be duly convicted. 

Early in 1847 the Proviso passed the House with the support among others of the newly elected Congressman from Illinois, Abraham Lincoln, but it failed in the Senate.

The Mexicans were too weak to withstand the enthusiastic American volunteer armies. They were beaten by General Zachary Taylor ('Old Rough and Ready') at Palo Alto, Resaca de la Palma. In September 1846 he took Monterey. In January 1847 Captain John C. Fremont accepted the surrender of the pro-Mexican resisters in California. On 23 February Santa Anna surrendered to Taylor at Buena Vista.  In the spring of that year General Winfield Scott ('Old Fuss and Feathers') began a mopping up operation that culminated in his capture of Mexico City on 13 September 1847. While Scott raised the American flag and entered the 'halls of Montezuma', Santa Anna and the remains of his army retreated to the suburb of Guadaloupe-Hidalgo.

In nearly three years of fighting almost 13, 000 Americans had died, either in combat or from disease, while 4,000 more were wounded.

The Treaty of Guadaloupe Hidalgo

On 2 February 1848 the Treaty of Guadaloupe Hidalgo ended the war. (It was ratified by the Senate on 10 March.) The treaty gave the U.S. undisputed control of Texas, established the U.S.-Mexican border of the Rio Grande, and ceded to the United States the present-day states of California, Nevada, Utah, New Mexico, most of Arizona and Colorado, and parts of Oklahoma, Kansas, and Wyoming. In return, Mexico received $15,000,000less than half the amount the U.S. had attempted to offer Mexico for the land before the opening of hostilities and the U.S. agreed to assume $3.25 million in debts that the Mexican government owed to U.S. citizens. By the Treaty, the United States acquired 600 million acres, most of them below the Mason-Dixon Line. It had increased its territory by 33 per cent.

The Mexican Cession

The results of the war

In the 1848 election the Democrats split on the slavery issue, allowing the successful general and slave-holding Whig, Zachary Taylor, to slip through in a narrow victory. (Abraham Lincoln, who had pledged to serve only one term, was out of Congress.) However, though the South seemed to have emerged the victor in the war, the expansion of the United States did not benefit the slave-holders, as the new territories were far more suitable for small farming than for slave plantations.  The South had never been more prosperous as cotton production and cotton profits reached a peak, but the Southern economy could not be exported. 

The war ensured that slavery was now at the top of the agenda. To

Henry David Thoreau

the New England moralist, Henry David Thoreau (1817-65) the war was obscene and he had refused to pay the taxes that were to finance it. Far from uniting Americans, the war had divided them as never before.

The Compromise of 1850

On 24 January 1848 gold was discovered on the banks of the Sacramento River in California. In December the outgoing President Polk announced that 'there's gold in them thar hills'.  The resulting gold rush was the greatest mass migration in American history.  During 1849 some 80,000 ‘Forty-niners’ reached California. By 1854 the number would top 300,000. Though some brought their slaves with them, most immigrants were not slave-holders and it was clear that California was not going to be a slave state. 

Though a slave-holder himself  (born in Virginia and raised in Kentucky) the new President, Zachary Taylor, had his eye on his re-election and he knew he could not win on southern votes alone. He needed to keep his support in New York and began to work closely with William Seward, one of the state’s senators, who was the leader of the Northern anti-slavery Whigs. The South became fearful that he would adopt the Wilmot Proviso. If this became law, they would lose their right to expand into the new territories. 

Henry Clay, the great fixer, managed to broker another compromise, the last of his political career. In January 1850 he put forward five proposals: 

1. California was to be admitted to the Union as a free state; 
2. New Mexico and Utah were to be organized as territories and to settle for themselves whether they should be slave or free; 
3. the boundary dispute between Texas and the United States was to be settled, and the Texas Republic’s debt was to be paid by Congress; 
4. the slave trade to be abolished in the District of Columbia; 
5. a new and stricter Fugitive Slave Law would replace that of 1793.

The ensuing debate was fierce. John C. Calhoun left his sickbed for the Senate chamber to urge urged the South to stand firm in defence of ‘Southern rights’, whatever the cost.  He died on 31 March and President Zachary Taylor on 9 July.  The young Illinois Senator Stephen A. Douglas spoke in favour of Clay’s proposals.

In September Millard Fillmore, the new President, signed the last of the five measures into law. 

Conclusion

Both Southerners and Northerners were prepared to compromise, and when it was accepted, it seemed as if normality had been restored. But, as subsequent events were to show, the price of this achievement was fearfully high. Those who had predicted that the Mexican War would only exacerbate the debate on slavery were to be proved right.