A few years ago, my models were saying the US looked a short, so I started to took for reasons for that to be the case. They were not too hard to find to be honest - the US corporate debt market looked a mess then, and it looks a mess now. But as learnt in the the GFC and Eurocrisis, debt markets can look insane for ages before they break. So lets look at the set up we have first, AND then how the downgrade of US sovereign debt could affect it.
