In discussions within the cabinet, Adams took the position that slavery restriction was unconstitutional and inconsistent with the Louisiana Purchase treaty. He continued to worry that Missouri would derail the Spanish treaty. The pressures mounted. He wrote in his diary that “this is a question between the rights of human nature and the Constitution of the United States. Probably both will suffer by the issue of the controversy.” During a gripping evening with a favorite colleague, Secretary of War John C. Calhoun, he came face to face with the emerging argument on behalf of the expansion of slavery, framing it as a positive good. The founding fiction that slavery would die of its own accord or wither from the end of the international slave trade had faded. Instead, Adams was forced to confront the fact that the cabinet colleague he most admired for his intelligence and devotion to the republic found a neo-colonial dependence on Great Britain preferable to a union in which slavery existed on sufferance, banned from the western future.

A few days later, Adams joined a cabinet majority advising the president that slavery restriction was, in fact, constitutional. His own position had flipped, but clarified. Slavery questions were restricted to the states where slavery existed; where it did not yet exist was a different matter. Missouri was ambiguous, so a deal was inevitable. “The fault is in the Constitution of the United States, which has sanctioned a dishonourable compromise with slavery,” he confided to his diary. The dishonor lay in its conflict with the Declaration of Independence, which had grounded the American Revolution in the consent of the governed. Yet now it was clearer than ever that “slave representation has governed the Union.” Maybe he should not have signed on to the compromise by failing to object in the cabinet meetings. Maybe he could propose a constitutional convention. In public, however, he stayed out of the line of fire.

1821 instead became the year of his most famous, defining statement of America’s exceptional identity, a July 4th address in Washington, D.C. that depicted “conquest and servitude” as “mingled up in every part of the social existence” of Great Britain. The rebellions of the 17th century indexed this British history. The settlement of New England, by contrast, which stood in for the entire nation, involved the purchase of Indian lands and a social compact “in which conquest and servitude had no part.” The Declaration of Independence represented a new epoch in history because it delegitimized foundings based “upon conquest.” The United States stood for natural and equal rights; “her glory is not dominion, but liberty.” This historical interpretation undergirded the Monroe Doctrine he developed: Europe must not interfere in the Americas because the U.S. “abstained from interference” abroad and refused the “Imperial diadem.” “Let us not go abroad in search of monsters to destroy,” he argued, as part of a long campaign to distance the U.S. from British-style imperialism that helped justify the American variety, in which expansionists like General Jackson displaced Indians by creating facts on the ground. Slavery had nothing to do with the meaning of America: a statement as true to his understanding of world history and the image he wished to project abroad as it was false to the reality at home.