Alexander von Humboldt paid a short visit to the United States at the end of his famous travels in the New World. In Philadelphia, he met the leading scientists of the country. More importantly, President Jefferson invited him to Washington where he supplied the government with the latest statistical and geographical facts about New Spain (Mexico). Jefferson appreciated Humboldt's scientific achievements; Humboldt found Jefferson's Notes on Virginia a model of how to describe a region. Between Humboldt and Albert Gallatin, the Swiss-born Secretary of the Treasury, a lasting friendship developed based on common interests, e.g., in monetary questions.

Humboldt's interests with regard to the U.S. were centered around three topics: the spread of slavery and its consequences in terms of the maintenance of the Union; the mining of gold as compared with Russia; the possibilities of building a canal between the Atlantic and Pacific oceans.

The debate over the slave question caused Humboldt to increase his criticism of American politics. He used his influence in Prussia to have a law passed that prohibited the ownership of slaves, thereby signaling his opposition to slavery to American leaders. In addition, Humboldt envisioned the improvement of international relations through free trade, and therefore favored projects such as an inter-oceanic canal. He never stopped admiring the achievements of the new nation in the sciences. That is why he liked to call himself “half an American.”