While the La Follette Seamen's Act of 1915, a landmark Progressive Era labor law, is often understood as the culmination of a heroic campaign for industrial efficiency, consumer safety, and workers' rights, its origins lie in the political milieu of the Spanish-American War. When the 1898 war exposed critical resource shortages in the domestic American maritime industry on which the US navy had traditionally depended in wartime, white seamen's unionists and their Democratic allies in the US Congress sought to turn concerns about maritime and naval weakness into a political strength. They stressed a nineteenth-century institution of flexible capacity, in which private industry had furnished the vessels and men that allowed the US navy to temporarily expand for war, to legitimate and pursue their legislative goals. Dominated by white, European-born immigrants jealous of their jobs and wages in a rapidly industrializing global industry hiring more nonwhite and low-wage seamen in foreign ports, the unions and their allies linked a nationalist politics of maritime and naval crisis to a nativist and racist politics of immigration and citizenship in order to drive Chinese and other Asian seamen from private and US military vessels in the Pacific. By 1905, changes in the industry, ship technologies, and naval manpower eroded the navy's older reliance on commercial vessels and seamen. Yet an imperial politics of flexible capacity left a mark on maritime labor reform and its crusade to exclude Chinese workers from American vessels that endured at the heart of the La Follette Act.