Well, it wasn't instant. It takes time to suck the value out of an economy, or for the effects to ripple through. I don't see how the French asset stripping (railway rolling stock) and punitive reparations wasn't the root of it. Obviously other stupidities compounded it. Today global interactions are faster. After WWII there was a different approach.
Certainly Germany / Austria / USA involved more than one issue. It's supposed to have started in USA 1929, but people seem to have many reasons / explanations as to why it happened.
Monetarists claim caused by the banking crisis that caused 1/3 of all banks to vanish, starting with failure of the "New York Bank of the United States" Why did these banks fail? Was any factor bad debts in Europe? Was the Gold Standard the reason the Feds didn't intervene?
Was the "free market" believers (esp Austrians) position really that intervention was evil and the "depression" would be good for the market?
Or were the debts that caused the initial banks in USA to fail purely domestic? (nearly 1000 USA banks failed in first 18 months?).
Certainly 2008 in Ireland was a mix of unsustainable property speculation, the Anglo Irish Bank, a Ponzi scheme getting nod from Regulator and thus other banks copying their unsustainable lending, I don't know how much USA Sub prime debt reselling was involved. Irish Nationwide was worst offender after Anglo Irish. It seems incredible that the Irish Government at first was only going to guarantee the Anglo Irish and Irish Nationwide, that would have destroyed all the "better" banks. As it was it should have been the other way round, rather than including them.