It used to be considered the Wall Street Crash of October 1929, but in fact the US was already in its third (or possibly fourth) month of recession and world output may have peaked still earlier, while there was little to suggest to contemporaries the severity or scope of the crisis until well into the second quarter of 1930, perhaps even later.

The Depression's origins remain hugely complex and debated, and there's no single cause. One candidate is the US Federal Reserve's raising of interest rates in January-July 1928 from 3½% to 5% which choked off loans to Europe (driving Germany already into recession by the end of the year) and made it harder for other countries to adopt monetary relaxation as the economic outlook deteriorated, and again in August 1929 to 6%, which doubtless added to the abruptness of the US downturn.

Another potential culprit is France's "hoarding" of monetary gold in 1928-32 as it quadrupled its reserves to $3.2bn, swallowing up almost the entire global increase in central bank stocks. The Fed too failed until 1931 to use its holdings to back new money to counter the fall in prices: US money supply even fell in April 1928-August 1929 despite annual real GDP growth approaching 10% (though from an initial level still below 1926).

If one were to ask why the recession became a lasting world Depression, another explanation is the June 1930 Smoot-Hawley tariff, which raised US import duties to unprecedented levels and worsened the contraction of international trade. But it too ultimately exacerbated weaknesses already evident in the world economy in the 1920s.

So the answer is that there's no easy answer. The Depression was a long time in the making and drew on multiple sources. The whole international economy had remained in disequilibrium after WW1, with primary producers at the mercy of low prices, Europe's powers unable to resume their former role, the US as yet unwilling to exchange its growing pre-eminence for leadership. The fatal spark may have been the gunshot at Sarajevo in June 1914.