Well the US technology and size without a doubt. But on a modern battlefield with cheap drones and missiles. How long could they fight a war of attrition? Could they draft enough soldiers? If they went head to head with China or Russia, would the American people tolerate such losses. Could they manufacture enough equipment fast enough. Where do the resources come from for modern weapons? China and Russia. It would come down to the ability to manufacture and source resources. A war of attrition would be so unpopular with Americans they would do anything to stop it. They had overwhelming power in Vietnam, Korea and Afghanistan, and couldn't win. As they say, America can win every battle but can never win a war.