Our purchase of foreign oil is a military strategy actually.
Let's say that doing things like fueling our rockets and moving our aircraft carriers across the oceans takes oil.
Let's say that every landmass on earth has oil reserves. Wouldn't that be a surprise?
Then let's say we buy and use everyone else's first.
What's the end strategic military result?
Well, foreign oil reserves are totally depleted and they CANNOT fuel rockets or steam warships. However, we still have our oil reserves that we haven't tapped yet - maybe because we claimed we were saving the polar bear. Maybe because whatever we claimed, we would up secure knowing that we're the last nation on earth that can steam an aircraft carrier or fuel ICBMs, and thus we stay secure.