Between October, 1847, and the following February the theme of the story underwent considerable alteration. By the latter date, as noted above, the National Era was advocating the absorption of Mexico, insisting that it would be free territory, and citing along with other evidence, Calhoun's opposition to annexation as proof that the anti-slavery interests had nothing to fear from extensive territorial acquisitions. In other words, the National Era was convinced that if there had been a "pro-slavery conspiracy" to acquire all Mexico, it could not realize its ends even though the whole country were annexed. This conviction seems to have come largely as a result of the propaganda, which was streaming from the northern expansionist press and the opposition of Calhoun.The editor probably reasoned that since Calhoun was opposing absorption the expansionists at the North must be correct. If the main body of the anti-slavery forces could be converted to this point of view, the movement for absorption which was growing rapidly at the time would doubtless become very strong indeed.
Care should be taken not to exaggerate the anti-slavery sentiment for all Mexico. It is evident that some such sentiment did exist, but there was not sufficient time for it to develop to significant proportions. The Treaty of Guadalupe-Hidalgo had already been signed in Mexico when the National Era took up the cry of all Mexico with or without the Wilmot Proviso. In a short while the war was over and whatever anti-slavery sentiment there was for all Mexico collapsed along with the general expansion movement. Had the war continued several months longer it is not improbable that increasing numbers from the anti-slavery camp would have joined forces with those who were demanding the acquisition of Mexico. Their action would have been based on the assumption that they were undermining the position of the pro slavery forces. It was, not to be expected that those abolitionists, and there were undoubtedly some, who were using the bogey of "extension of slavery" to cover up other reasons for opposition to annexation, would have ever become convinced of the error of their ways. They would hold on to their pet theory to the bitter end.
To summarize briefly what seem to be the conclusions to be drawn from this study, it might be said that the chief support for the absorption of Mexico came from the North and West and from those whose pro-slavery or anti-slavery bias was not a prime consideration. In quarters where the attitude toward slavery was all-important there was, contrary to the accepted view, a "pro-slavery conspiracy" to prevent the acquisition of all Mexico and the beginnings of an "anti-slavery conspiracy" to secure all the territory in the Southwest that happened to be available. Behind both these movements was a belief that expansion would prove injurious to the slavery interest. Had the war continued much longer the two movements, would probably have developed strength and have become more easily discernible. Lack of time for expansionist sentiment to develop was the chief cause of this country's, failure to annex Mexico in 1848. Even as it was, however, there might have been sufficient demand for annexation in February and March, 1848, to have wrecked the Treaty of Guadalupe-Hidalgo had it not been for the opposition of pro-slavery Democrats led by Calhoun. Their attitude divided the party committed to expansion in the presence of a unified opposition. Whatever the motives which may be attributed to Calhoun and his friends, the fact remains that those who feel that the absorption of Mexico in 1848 would have meant permanent injury to the best interests of the United States, should be extremely grateful to those slaveholders. To them not a little credit is due for the fact that Mexico is to-day an independent nation.
