This dissertation analyzes mobility and power in an age of expansionary imperialism and accelerated globalization. Based on archival research in France, the U.K., and Vietnam, "In-between Empires" reconstructs the social histories of steamship voyages along the Trans-Suez maritime highways connecting France to its Indo-Pacific empire. In so doing, the dissertation reinterprets the rise of steam-powered imperialism between the 1830s and the 1930s. Much historiography has assumed that steamships, port complexes, and maritime highways represented unproblematic pathways through which imperial sovereignty flowed as smoothly as information through an underwater cable. Reifying a meta-narrative in which space and time were swiftly and uniformly annihilated in the latter-19th century, such an interpretation risks implying that European states, once equipped with coal-burning steamships, moved people and cargo reflexively and without resistance across the thousands of kilometers separating empire and metropole. This dissertation, by contrast, demonstrates that the steamship highways crisscrossing the Suez Canal were contested borderlands in which empire was made and unmade. Juxtaposing the everyday life of crowded steamships against the inter-imperial politics of a vast maritime route connecting European metropoles to empire in Africa, Asia, and Oceania, the dissertation argues that French imperial sovereignty took shape in the struggle to control people in motion and govern a trans-imperial highway. From the militant demands of a multi-racial maritime workforce to the elite networking of an interoceanic commuter class, a host of overlooked actors jockeyed for influence over the highways of imperialism. As a result, at the apex of Europe’s “New Imperialism” and a global age of steam, seemingly routine transit was fraught with conflict and filled with encounters, many of which called into question the stability of colonial racial hierarchies and the ideology of civilization itself. In the interstices of empire, moreover, the dilemmas of daily life – establishing legal jurisdiction, for instance, or following rules for disease control – produced dramatic experiments in inter-imperial governance while enlivening the tensions between nation, empire, and capital.