Popular accounts of the Halifax Explosion of 1917 have placed it in a resolutely nationalist context. But starting from the international ownerships and destinations of the ships that sparked it, the explosion was a transnational event. This article explores how people, money, and ideas crossed and recrossed the border. First, in-kind and monetary relief flowed quickly from the United States, Britain, and Newfoundland. Second, Halifax became a destination for a growing international community of experts in disaster response, as relief experts from New York, Boston, Winnipeg, and elsewhere in North America converged on the city. Finally, survivors used their transnational community of friends and relatives to build political power over the relief process. Migrants living in "the Boston States" created a transnational polity that pressured relief authorities to give more money to their kin still in Halifax. These transnational communities-of international experts and migrant families-helped create a Canada-US relationship from the bottom.When two ships collided in Halifax Harbor in December 1917, they sparked what has been called the largest manmade explosion before the atomic bomb, killing about 2000 people and leaving tens of thousands more maimed, homeless, or jobless. 1 The very fact of the explosion stemmed from Halifax's roles not just within Canada but also in the broader world: a French owned and crewed ship, en route from New York City to Brest, sailed to Halifax, under orders from the British Admiralty, where it collided with a Norwegian ship under contract to Belgian Relief. Subsequent events emphasize that the explosion was a transnational event. First, in-kind and monetary relief flowed quickly from the United States, Britain, and Newfoundland. Second, Halifax became a destination for a growing international community of experts in disaster response, as relief experts from New York, Boston, Winnipeg, and elsewhere in North America converged on the city. They brought with them a particular ideology of professionalized relief developed and spread by the American Red Cross. Finally, survivors used their transnational community of friends and relatives to build political power over the relief process. Migrants living in "the Boston States" created a transnational polity that pressured relief authorities to give more money to their kin still in Halifax. Nova Scotia's professional and lay connections were strongest with Massachusetts, and these transnational communities-of international experts and migrant families-spread money, ideas, and ideology across the border and helped create a Canada-US relationship from the bottom.