The start of the Great Depression is commonly assumed to be the October 1929 stock market crash in the United States. It didn't really become the Great Depression, however, unti 1931, when Austria's Creditanstalt bank desperately needed injections of capital. Essentially, neither France nor England were willing to help unless Germany honored its reparations payments, and the United States refused to help unless France and the UK repaid its World War I debts. Neither of these demands was terribly reasonable, and the result was a wave of bank failures that spread across Europe and the United States.

http://drezner.foreignpolicy.com/posts/2011/08/09/is_it_1931_yet