The peak of U.S. power was after World War II, when it had literally half the world's wealth. But that naturally declined, as other industrial economies recovered from the devastation of the war and decolonization took its agonizing course. By the early 1970s, the U.S. share of global wealth had declined to about 25%
There was also a sharp change in the U.S. economy in the 1970s, towards financialization and export of production. A variety of factors converged to create a vicious cycle of radical concentration of wealth, primarily in the top fraction of 1% of the population -- mostly CEOs, hedge-fund managers, and the like. That leads to the concentration of political power, hence state policies to increase economic concentration: fiscal policies, rules of corporate governance, deregulation, and much more. Meanwhile the costs of electoral campaigns skyrocketed, driving the parties into the pockets of concentrated capital
Is the World Too Big to Fail? The Contours of Global Order, Truthout, Noam Chomsky, April 21, 2011.