US federal debt was largely domestically held. Also 'most European countries had almost fully recovered by the mid-50s' seems quite wrong to me as a European. Certainly, they had functional economies again and cleaned up the mess of the war, but that's hardly a full recovery. European countries were paying off war debt far longer than that too. The US had a huge economic advantage for several decades.

Things like price controls and the GI bill and the Cold War all helped in their way, but I'm not sure that that's a formula you can deploy going forward.