By examining a hugely significant, yet overlooked, diplomatic incident, known as the Martin Koszta Affair (1853), this article offers a new interpretation of nationalism in the antebellum United States. Martin Koszta was a Hungarian refugee who had declared his intention of becoming an American citizen after the failed revolutions of 1848. But, before the process of naturalization was complete, Koszta returned to Europe, where he was captured by Austrian authorities off the coast of Turkey. After an American naval captain rescued Koszta, the Democratic administration negotiated the Hungarian's safe return to the United States, arguing his "natural right" to expatriation trumped Austria's doctrine of perpetual allegiance, and that Koszta was entitled to U.S. protection because of his "domiciled" status. This article shows that a faction within the Democratic Party, known as the Young America movement, advocated an even more radical justification for Ingraham's actions, claiming the incident had set a precedent for protecting "inchoate" citizens beyond the borders of the Union. In doing so, they championed a form of nationalism that collapsed the boundary between natural and citizenship rights, and expanded the remit of American foreign policy. This response complicates the emphasis on sectional divisions that dominates the scholarship in this period. In fact, there was an intra-sectional conflict about the basis of nationality that hinged on the tension between a legalistic definition of citizenship and a natural rights definition, with significant implications for the way Americans perceived the Union's role in the world.