In 1829, due to the large influx of American immigrants, the Americans outnumbered Mexicans in the Texas territory. The Mexican government decided to bring back the property tax, increase tariffs on US shipped goods, and the prohibition of slavery. Many, if not all, settlers refused to obey, which led to Mexico closing Texas to additional immigration from the Americas. However, citizens from the southern states continued to flow into the Texas territory.
...enforce the Mexican laws in Texas, which led Stephen Austin calling Texas to arms and American immigrants in Texas declaring independence from Mexico in 1836...
The Mexican government had long warned the United States that annexation would mean war. Because the Mexican congress had refused to recognize Texan independence, Mexico saw Texas as a rebellious territory that would be retaken.
Southern Democrats, animated by a popular belief in Manifest Destiny, supported it in hopes of adding territory to the South and avoiding being outnumbered by the faster-growing North.
John O'Sullivan, the editor of the "Democratic Review", coined this phrase in its context, stating that it must be "Our manifest destiny to overspread the continent alloted by Providence for the free development of our yearly multiplying millions.
Joshua Giddings led a group of dissenters in Washington D.C. He called the war with Mexico "an aggressive, unholy, and unjust war," and voted against supplying soldiers and weapons.
Fellow Whig Abraham Lincoln contested the causes for the war and demanded to know exactly where Thornton had been attacked and American blood shed. "Show me the spot," he demanded.
Former President John Quincy Adams also expressed his belief that the war was primarily an effort to expand slavery in a speech he gave before the House on May 25, 1846.
President Ulysses S. Grant, who as a young army lieutenant had served in Mexico under General Taylor, recalled in his Memoirs, published in 1885, that:
“ For myself, I was bitterly opposed to the measure, and to this day regard the war, which resulted, as one of the most unjust ever waged by a stronger against a weaker nation. It was an instance of a republic following the bad example of European monarchies, in not considering justice in their desire to acquire additional territory.
described the war as "Feculent, reeking Corruption" and "one of the darkest scenes in our history - a war forced upon our and the Mexican people by the high-handed usurpations of Pres't Polk in pursuit of territorial aggrandizement of the slave oligarchy."[
