Petrodollar. Denominating oil prices in other currencies would cause oil producing countries to stop importing dollars (accepting payments in dollars), which would cause all those dollars to return back to the US and cause a hyperinflation. Furthermore, as if that wasn't bad enough, the Fed wouldn't be able to print dollars at the rate they currently do (the money would have nowhere to go), which would cause the country to default because the government wouldn't be able to repay its obligations.

The war in Iraq and the coup in Libya were done purely out of economic self-preservation, and not because a few savages with AK47s would pose a serious threat to the western world.