The underlying premise is the usual, that the U.S. is the creator and supporter of the "liberal world order" maintained through U.S. military power. To his credit, Kagan does not have a Pollyanna view of the world and cites examples of U.S. "errors" and "mistakes" that have caused more harm than good, but even then the underlying implication is that they were mainly mistakes for a good cause: "the liberal democratic capitalist" world order.
At first, there is little definition of what this "liberal" order consists of. Later he provides this: "rules-based, free trade, market economy…undergirded by American power." Yes, there are rules, consistently abrogated by the U.S., especially in areas of international and humanitarian and war law, but also domestic laws that assume extraterritoriality and extrajudicial means to control the rest of the world (probably an even split between military and economic 'rules').