The collapse of the USD? ~$7T USTs need to be rolled over before Christmas - if the interest rate the market demands is significantly higher then the US government inches closer to insolvency- where the increasing cost of servicing your already massive debt load triggers a runaway reduction in investors prepared to put their capital at risk. In such a scenario the FED could step in and buy USTs but this again could still trigger panic in UST holders and a sell off that would be an effective run on the USD.