‘Linkage” was a term introduced to American diplomacy by Henry Kissinger at the outset of the Nixon administration. Linkage, Kissinger wrote in his memoir, “White House Years,” could be an explicit gambit — for example, making “progress in settling the Vietnam War . . . a condition for advance in areas of interest to the Soviets, such as the Middle East, trade, or arms limitation.” But linkage was also an implicit reality in an increasingly interdependent world.