This article examines the prelude to, and machinations surrounding, the arrest, trial, and expulsion from America of Charles Rowcroft, the British consul in Cincinnati. Rowcroft's difficulties were a direct consequence of the conniving of Irish-American nationalists in the region during the Crimean War. The article places these events in Cincinnati against a backdrop of intense Anglo-American diplomatic distrust. It also highlights the exaggerated Hibernophobic response of some British officials in the United States. A study of Irish-American nationalism during the 1850s, bridging the historical and historiographical gap between the 1848 Young Ireland rebellion and the beginnings of Fenianism, has long been wanting. This article is a first, important step toward filling that void, elucidating the hitherto hidden extent of Irish-American agitation during the Crimean War.