This paper examines how financial markets responded to the longest circuit breaker in American financial history: the four-month suspension of trading on the New York Stock Exchange following the outbreak of World War I. The suspension that began on July 31, 1914 fostered a substitute trading forum called the New Street market. Trading on New Street began almost immediately and offered economically meaningful liquidity services despite its impaired price transparency. A simple cross-sectional model of bid–ask spreads on New Street demonstrates that New Street liquidity responded to economic incentives. New Street's success implies that, from a public policy perspective, expensive back-up trading facilities are not required to preserve liquidity during a trading suspension in established markets. Back-up records of share ownership and transfer facilities, however, are crucial to maintaining liquidity.