Nit-pick: The economic historian Niall Ferguson argued that US fiscal deficits could lead at some point in time to a rapid collapse in the United States economy, noting “most imperial falls are associated with fiscal crisis”.  Such a crisis would drag down every other economy, including those of China and Saudi Arabia.
Note though that the difference between US and Greece is humongous in this regard. The US actually can’t possibly be forced to default on payments in its own currency (although it can choose to do so for political reasons) as there it has sovereign control the currency in question.
Greece on the other hand can indeed default. It has no such control over the currency, and therefore works more like a household or a firm.
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