1 Trade Wars and the Slump # James Foreman-Peck * , Andrew Hughes Hallett + and Yue Ma § Abstract Simulated optimum tariff policies to achieve plausible government targets show objective reasons for the failure of international trade cooperation during the Slump of 1929 to 1933. For the largest players, the US and UK, benefits from foreign cooperation were small or negative. France and Germany would have been the principal beneficiaries of international cooperative policies. Since cooperation on one issue, tariffs, was difficult, cooperation on many  through the cross-issue bargaining attempted at the 1933 London Conference- was well nigh impossible. Optimum coordination across policy instruments within one country, on the other hand, would have yielded high returns for policy. A corollary is that lack of internal coordination (poor domestic policies) was a more important cause of the Great Depression than failure to harmonise policies internationally. November 2006 # Versions of this paper have been given at a conference at Trinity College, Dublin and at the Second German Cliometrics Conference, Tuebingen. We are grateful for the comments of participants and of anonymous referees of this journal. The original modelling was financed by ESRC grant R000 23 1534. * Cardiff University, Wales. + George Mason University, Fairfax, Virginia. § Lingnan University, Hong Kong.