If we wish to know, we can. An innovative approach comes from Roger Stern, an economic geographer at Princeton University who in April published a peer-reviewed study on the cost of keeping aircraft carriers in the Persian Gulf from 1976 to 2007. Because carriers patrol the gulf for the explicit mission of securing oil shipments, Stern was on solid ground in attributing that cost to oil. He had found an excellent metric. He combed through the Defense Department’s data — which is not easy to do because the Pentagon does not disaggregate its expenditures by region or mission — and came up with a total, over three decades, of $7.3 trillion. Yes, trillion.