Whenever there has been a world crisis, it has been typical for foreigners to seek the safety of the US dollar. A common means to accomplish that has been to buy US Treasury debt. The typical example over the past couple of years is that the demand for US Treasury debt has increased at the various peaks of crises among the European Union nations such as Greece, Ireland, Spain, Portugal, and the like.
As a result of these surges in demand, the US Dollar Index, which reflects the value of the dollar against other currencies, tends to increase.