This paper examines how class conflict affected US imperial expansion between 1898 and 1906. It focuses on West-Coast-based white merchant sailors and relies on union publications, legislative records, and congressional testimony to reveal how domestic class conflict shaped the boundaries, both internally and externally, of the emerging US empire. The struggle of the sailors’ unions over these imperial boundaries illustrates the real-life consequences they held for working people. These were not just abstractions. These lines often determined the type of labor systems under which workers would toil. Specifically, this article centers on the American Federation of Labor's successful effort to apply the Chinese Exclusion Act to the United States’ empire on the Pacific in 1902 as well as the Sailor's Union of the Pacific's unsuccessful attempt to apply exclusion to US-flagged merchant vessels. With that in mind, I argue that the line between domestic and foreign or nation and empire was a contested space of racially inflected class conflict. For white working people, the most pertinent question in the aftermath of the Spanish American War was not, does the constitution follow the flag? But rather, does exclusion follow the flag?