Texas was part of Mexico until American newcomers decided to make it their own country. Whether the new Republic of Texas would join the Union and whether it would be a slave state were much-debated issues until 1845 when Congress agreed to admit Texas as the 18th state. We’d soon be at war with Mexico. (American politics hasn’t totally changed since 1845, although the Democrats eventually switched sides and became less warlike.)
From “Heirs of the Founders: The Epic Rivalry of Henry Clay, John Calhoun and Daniel Webster” by H. W. Brands:
Four months [after Texas joined the Union], James Polk made prophets of Henry Clay and Daniel Webster by commencing a war with Mexico. Polk’s war was a land grab wrapped in self-defense. Texas entered the Union with its southern boundary in dispute. The United States claimed the Rio Grande as the border; Mexico claimed the Rio Nueces, more than a hundred miles to the north. Mexico nominally claimed the rest of Texas as well, never having acknowledged the loss of its rebellious province. But though it responded to the American annexation of Texas by severing relations with the United States, it took no military action to challenge the new regime on its northern frontier. This frustrated Polk.
The president’s expansionist appetite grew with the eating; not content with depriving Mexico of Texas, Polk coveted California as well. He attempted to purchase California, but the Mexican government rebuffed him. Polk then sought a pretext for declaring war on Mexico. He sent troops to the disputed strip between the rivers, hoping to goad the Mexicans to attack.
Weeks went by and the Mexicans refused to take the bait. Polk, more vexed than ever, prepared a war message for Congress, in which he blamed the Mexicans for insults and injuries against American honor and interests. It was a flimsy document, as Polk himself recognized, but he was determined to have California and its Pacific harbors, by whatever means necessary.
Then, just as he was about to transmit his message to Congress, he received news that Mexican troops had finally engaged the Americans. “After reiterated menaces, Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil,” Polk told Congress. “War exists, and notwithstanding all our efforts to avoid it, exists by the act of Mexico itself.” For emphasis the president added, “The two nations are now at war.”
John Calhoun begged to differ. Polk wanted Congress simply to endorse his assertion that war existed and give him authority to prosecute it. Calhoun wasn’t going to be stampeded into anything. “The question now submitted to us is one of the gravest character, and the importance of the consequences which may result from it we cannot now determine,” he told the Senate. “The president has announced that there is war; but according to my interpretation, there is no war according to the sense of our Constitution.”
Calhoun didn’t challenge Polk’s account of the attack on American forces. Nor did he question Polk’s authority to resist and repel such attacks. But he distinguished hostilities from war. “It is our sacred duty to make war,” he told his fellow senators, “and it is for us to determine whether war shall be declared. If we have declared war, a state of war exists, and not till then.”
Calhoun succeeded in slowing the rush to war, but not by much. Congress debated the president’s request, with most of the negative comments coming from the Whigs [Polk, like Andrew Jackson, was a Democrat].
Some asked whether Polk had done all he could to avoid armed conflict; their strong implication was that he had not. A few went so far as to charge Polk with provoking the war. “This war was begun by the president,” Garrett Davis, a Kentucky Whig, told the House. Some inquired whether the Mexican attack, if it indeed had occurred as the president said, had been authorized by the Mexican government. Still others rejected Polk’s assertion that the soil on which the blood had been shed was American. Some said flatly that it was Mexican; others remarked that ownership was still in dispute.
But Polk knew the American political mind better than the dissenters did. He understood that the shedding of American blood—under whatever circumstances—created an irresistible impulse toward war. A negative vote could be characterized as an unpatriotic vote, and no lawmaker lightly risked that. The few surviving former Federalists remembered how their party had wrecked on its opposition to the War of 1812. In the end scarcely a dozen Whigs refused the president’s request. John Calhoun haughtily abstained.
Unquote.
Ulysses S. Grant fought in the Mexican War as a young lieutenant. Years later, he called it “one of the most unjust ever waged by a stronger against a weaker nation”.
Whereof One Can Speak 🇺🇦 
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