First, Americans are not very comfortable with the moral and reputational costs of realist statecraft. Many Americans — particularly those who aspire to its leadership classes — are cringing at Trump’s comments about Saudi Arabia or North Korea.
The idealist streak in American foreign policy also takes deep inspiration from America’s success in the 20th century, in which America defeated despotisms three times over, in two epic world wars and in the Cold War. America’s entry into World War I and World War II could never have been sold to the country in terms of snatching the financial and naval primacy of world leadership from the British Empire — one of the real and most enduring practical effects of the war. If foreign-policy realists try to justify themselves as the party that will safely and effectively manage America’s declining influence in world affairs, they will make themselves detestable.
The idealist streak in American foreign policy is also the product of America’s inherited Protestant imagination. When you hear even supposedly secular people talk about the progress of mankind, or the arc of history, you are hearing a peculiar form of America’s liberal millenarianism. This moral worldview makes it harder for Americans to intervene in our interests and then remain indifferent to the subjects of our intervention. As America becomes less Protestant, this wellspring of our idealism in foreign affairs will begin to dry up. But I expect we have a long way to go before it is emptied.