The British Minister declined this proposition in the part that carried the line to the ocean, but offered to continue it from the summit of the mountains to the Columbia River, a distance of some three hundred miles, and then follow the river to the ocean. This was declined by Mr. Calhoun. The President had declared in his inaugural address in favor of the 54–40 line. He was in a dilemma; to maintain that position meant war with Great Britain; to recede from it seemed impossible. The proposition for the line of 49 degrees having been withdrawn by the American government on its non-acceptance by the British, had appeased the Democratic storm which had been raised against the President. Congress had come together under the loud cry of war, in which Mr. Cass was the leader, but followed by the body of the democracy, and backed and cheered by the whole democratic newspaper press. Under the authority and order of Congress notice had been served on Great Britain which was to abrogate the joint occupation of the country by the citizens of the two powers. It was finally resolved by the British Government to propose the line of 49 degrees, continuing to the ocean, as originally offered by Mr. Calhoun; and though the President was favorable to its acceptance, he could not, consistently with his previous acts, accept and make a treaty, on that basis. The Senate, with whom lies the power, under the constitution, of confirming or restricting all treaties, being favorable to it, without respect to party lines, resort was had, as in the early practice of the Government, to the President, asking the advice of the Senate upon the articles of a treaty before negotiation. A message was accordingly sent to the Senate, by the President, stating the proposition, and asking its advice, thus shifting the responsibility upon that body, and making the issue of peace or war depend upon its answer. The Senate advised the acceptance of the proposition, and the treaty was concluded.

The conduct of the Whig Senators, without whose votes the advice would not have been given nor the treaty made, was patriotic in preferring their country to their party—in preventing a war with Great Britain—and saving the administration from itself and its party friends.

The second session of the 29th Congress was opened in December, 1847. The President’s message was chiefly in relation to the war with Mexico, which had been declared by almost a unanimous vote in Congress. Mr. Calhoun spoke against the declaration in the Senate, but did not vote upon it. He was sincerely opposed to the war, although his conduct had produced it. Had he remained in the cabinet, to do which he had not concealed his wish, he would, no doubt, have labored earnestly to have prevented it. Many members of Congress, of the same party with the administration, were extremely averse to the war, and had interviews with the President, to see if it was inevitable, before it was declared. Members were under the impression that the war could not last above three months.

The reason for these impressions was that an intrigue was laid, with the knowledge of the Executive, for a peace, even before the war was declared, and a special agent dispatched to bring about a return to Mexico of its exiled President, General Santa Anna, and conclude a treaty of peace with him, on terms favorable to the United States. And for this purpose Congress granted an appropriation of three millions of dollars to be placed at the disposal of the President, for negotiating for a boundary which should give the United States additional territory.

While this matter was pending in Congress, Mr. Wilmot of Pennsylvania introduced and moved a proviso, “that no part of the territory to be acquired should be open to the introduction of slavery.” It was a proposition not necessary for the purpose of excluding slavery, as the only territory to be acquired was that of New Mexico and California, where slavery was already prohibited by the Mexican laws and constitution. The proviso was therefore nugatory, and only served to bring on a slavery agitation in the United States. For this purpose it was seized upon by Mr. Calhoun and declared to be an outrage upon and menace to the slaveholding States. It occupied the attention of Congress for two sessions, and became the subject of debate in the State Legislatures, several of which passed disunion resolutions. It became the watchword of party—the synonym of civil war, and the dissolution of the Union. Neither party really had anything to fear or to hope from the adoption of the proviso—the soil was free, and the Democrats were not in a position to make slave territory of it, because it had just enunciated as one of its cardinal principles, that there was “no power in Congress to legislate upon slavery in Territories.” Never did two political parties contend more furiously about nothing. Close observers, who had been watching the progress of the slavery agitation since its inauguration in Congress in 1835, knew it to be the means of keeping up an agitation for the benefit of the political parties—the abolitionists on one side and the disunionists or nullifiers on the other—to accomplish their own purposes. This was the celebrated Wilmot Proviso, which for so long a time convulsed the Union; assisted in forcing the issue between the North and South on the slavery question, and almost caused a dissolution of the Union. The proviso was defeated; that chance of the nullifiers to force the issue was lost; another had to be made, which was speedily done, by the introduction into the Senate on the 19th February, 1847, by Mr. Calhoun of his new slavery resolutions, declaring the Territories to be the common property of the several States; denying the right of Congress to prohibit slavery in a Territory, or to pass any law which would have the effect to deprive the citizens of any slave State from emigrating with his property (slaves) into such Territory. The introduction of the resolutions was prefaced by an elaborate speech by Mr. Calhoun, who demanded an immediate vote upon them. They never came to a vote; they were evidently introduced for the mere purpose of carrying a question to the slave States on which they could be formed into a unit against the free States; and so began the agitation which finally led to the abrogation of the Missouri Compromise line, and arrayed the States of one section against those of the other.

The Thirtieth Congress, which assembled for its first session in December, 1847, was found, so far as respects the House of Representatives, to be politically adverse to the administration. The Whigs were in the majority, and elected the Speaker; Robert C. Winthrop, of Massachusetts, being chosen. The President’s message contained a full report of the progress of the war with Mexico; the success of the American arms in that conflict; the victory of Cerro Gordo, and the capture of the City of Mexico; and that negotiations were then pending for a treaty of peace. The message concluded with a reference to the excellent results from the independent treasury system.

The war with Mexico was ended by the signing of a treaty of peace, in February, 1848, by the terms of which New Mexico and Upper California were ceded to the United States, and the lower Rio Grande, from its mouth to El Paso, taken for the boundary of Texas. For the territory thus acquired, the United States agreed to pay to Mexico the sum of fifteen million dollars, in five annual installments; and besides that, assumed the claims of American citizens against Mexico, limited to three and a quarter million dollars, out of and on account of which claims the war ostensibly originated. The victories achieved by the American commanders, Generals Zachary Taylor and Winfield Scott, during that war, won for them national reputations, by means of which they were brought prominently forward for the Presidential succession.

The question of the power of Congress to legislate on the subject of slavery in the Territories, was again raised, at this session, on the bill for the establishment of the Oregon territorial government. An amendment was offered to insert a provision for the extension of the Missouri compromise line to the Pacific Ocean; which line thus extended was intended by the amendment to be permanent, and to apply to all future territories established in the West. This amendment was lost, but the bill was finally passed with an amendment incorporating into it the anti-slavery clause of the ordinance of 1787. Mr. Calhoun, in the Senate, declared that the exclusion of slavery from any territory was a subversion of the Union; openly proclaimed the strife between the North and South to be ended, and the separation of the States accomplished. His speech was an open invocation to disunion, and from that time forth, the efforts were regular to obtain a meeting of the members from the slave States, to unite in a call for a convention of the slave States to redress themselves. He said: “The great strife between the North and the South is ended. The North is determined to exclude the property of the slaveholder, and, of course, the slaveholder himself, from its territory. On this point there seems to be no division in the North. In the South, he regretted to say, there was some division of sentiment. The effect of this determination of the North was to convert all the Southern population into slaves; and he would never consent to entail that disgrace on his posterity. He denounced any Southern man who would not take the same course. Gentlemen were greatly mistaken if they supposed the Presidential question in the South would override this more important one. The separation of the North and the South is completed. The South has now a most solemn obligation to perform—to herself—to the constitution—to the Union. She is bound to come to a decision not to permit this to go on any further, but to show that, dearly as she prizes the Union, there are questions which she regards as of greater importance than the Union. This is not a question of territorial government, but a question involving the continuance of the Union.” The President, in approving the Oregon bill, took occasion to send in a special message, pointing out the danger to the Union from the progress of the slavery agitation, and urged an adherence to the principles of the ordinance of 1787—the terms of the Missouri compromise of 1820—as also that involved and declared in the Texas case in 1845, as the means of averting that danger.

The Presidential election of 1848 was coming on. The Democratic convention met in Baltimore in May of that year; each State being represented in the convention by the number of delegates equal to the number of electoral votes it was entitled to; saving only New York, which sent two sets of delegates, and both were excluded. The delegates were, for the most part, members of Congress and office-holders. The two-thirds rule, adopted by the previous convention, was again made a law of the convention. The main question which arose upon the formation of the platform for the campaign, was the doctrine advanced by the Southern members of non-interference with slavery in the States or in the Territories. The candidates of the party were, Lewis Cass, of Michigan, for President, and General Wm. O. Butler, of Kentucky, for Vice-President.

The Whig convention, taking advantage of the popularity of Genl. Zachary Taylor, for his military achievements in the Mexican war, then just ended; and his consequent availability as a candidate, nominated him for the Presidency, over Mr. Clay, Mr. Webster and General Scott, who were his competitors before the convention. Millard Fillmore was selected as the Vice-presidential candidate.