sentences:  Zbigniew Brzezinski, Jimmy Carter's national security adviser, said he thought there was no "single piece of paper" that represents the Bush doctrine.Experts on geopolitical strategy note that Halford Mackinder's theories in "The Geographical Pivot of History" about the "Heartland" and world resource control are still as valid today as when they were formulated.In his 2007 book "In the Defense of the Bush Doctrine", Robert G. Kaufman wrote