The process of leaving a customs union is immensely complicated and time-consuming, as some politicians have belatedly discovered in post-Brexit Britain. Similarly, the thirteen colonies’ decision to depart from the customs union of the British mercantilist state led to a realization that political independence did not sever economic ties across the Atlantic with the new country’s major trading partner and financier. Despite its defeat in 1783, the British Empire remained expansive and dynamic, and American businessmen needed to maintain links. And not unlike the situation in post-Brexit Britain, the issue of citizenship hung in the balance. The more common story is how British immigrants became American despite their supposed “perpetual allegiance” to the sovereign. Yet, another issue, which is at the core of Tuffnell’s book, is that the continued economic ties across the Atlantic led to expatriate communities of Americans whose status and loyalty remained in question. Were they informal ambassadors who could leverage British power to American economic interests, or were they dangerously de-nationalized vectors of antirepublican contagion who were evading the obligations of citizenship and even serving as agents of re-colonization?At the outbreak of the American Civil War, two main expatriate groups were in Britain—about 2,800 Americans in Liverpool and 1,910 in London. The cotton trade of Liverpool linked the slave south with Lancashire mill towns; the credit and capital of London financed trade and investment in the growing economy of the United States. By the end of the century, the expatriate community in Liverpool fell to fewer than 1,000; the cotton trade could be managed by cable. By contrast, the expatriate community in the City of London rose to around 5,000 to exploit the opportunities of the world’s major financial center. Tuffnell is interested in the economic activities and associations of the two communities, even more in their social lives—their clubs and celebrations of American identity that could also foster American interests through engagement with British elites.American slavery and the Civil War created tensions in Britain. The Liverpool merchants expected support from English aristocrats and Lancashire mill owners in defense of the Confederacy. However, their networks were shallower than the social contacts that the Union’s State Department established in London. The rift was healed in the later nineteenth century through a shared “Anglo-Saxondom”—founded on racial superiority—that fostered cooperation between two kindred empires, despite the counter-charge that this form of cosmopolitanism subverted American nationalism. The debate between insular nationalism and internationalism continues in both countries to this day.Tuffnell’s analysis is compelling and well told, with generous details and incidents to inform and entertain. His main theme of the identity of expatriates in Britain is certainly worth exploring, though it has gaps. The issue of expatriate identity extends beyond the business communities in London and Liverpool to the world of culture and high society. Tuffnell mentions the “gilded prostitution” of American heiresses marrying British aristocrats—for instance, the marriage of Mary Leitner of Chicago to George Nathaniel Curzon or that of Consuelo Vanderbilt to Charles Spencer-Churchill, the duke of Marlborough. He points out that these marriages intimated the kind of international “special relationship” to which Winston Spencer Churchill alluded in his A History of the English-Speaking Peoples (New York, 1956–1958), 4 v. He fails to mention, however, expatriates like Henry James or John Singer Sargent, who inhabited a milieu of social and cultural, rather than strictly business, exchange.Tuffnell also could have deepened his focus on the business communities in London and Liverpool by considering the network of interactions in the ports of the eastern seaboard. Liverpool, London, and, for that matter, Bristol and Glasgow were part of a North American world that including Baltimore, Boston, New York, and New Orleans in which firms operated on both side of the Atlantic, whether those of the Barings, the Brown Brothers, or George Peabody. They were joined in the later nineteenth century by other businessmen who operated within the capacious limits of the British Empire, such as the mining engineer Herbert Hoover or Chester Beatty, the “king of copper,” who transformed from a New Yorker into a British knight and ultimately an honorary citizen of Ireland.Tuffnell’s study is fascinating as it stands. It is no criticism to say that it is fertile soil for future work that could extend the coverage of the expatriate community into culture and society, add links to the eastern seaboard of the United States, and move ahead to the continued connections between American business and British imperialism and, more recently, the American influence on the City of London.