Abstract: Britain abolished its transatlantic slave trade in 1807 after MPs with ties to the Caribbean economy abandoned their opposition. What explains this shift? I argue that abolition was a protectionist policy that shielded established Caribbean sugar producers from new competitors. A general framework identifies the conditions under which entry makes it optimal for incumbent producers to support regulations that also impose costs on themselves. British conquests during the Napoleonic Wars brought new land-rich but labor-poor sugar colonies into the protected imperial market, while the trade war with France closed continental European markets to British colonial sugar. This intensified competition within the empire, prompting established planters to support abolition in an attempt to slow down the economic expansion of their new labor-poor rivals. Three findings support this account. First, newly digitized data show that parliamentary support for abolition shifted between December 1806 and February 1807, following the onset of a trade war with France. Second, a semi-structural model estimated using an instrumental-variable strategy shows that sugar prices would have been substantially lower had the slave trade continued. Third, archival evidence confirms that planters regarded competition from the new colonies and the loss of European markets as their principal economic threats. These findings show how geopolitical shocks can reshape markets, induce elites to abandon policies they once defended, and create unexpected coalitions for institutional change.