This is a paradox, but it is understandable. Mr. Obama is channeling the voters. Just as Americans want to eliminate the federal budget deficit without cutting Social Security or Medicare, they want a more peaceful and democratic world with less heavy lifting from the U.S. Who wouldn’t want an easier life in a nicer world?

Unfortunately, it’s hard to transform and democratize the world while saving money and reducing overseas commitments. A world based more on the rule of law and less on the law of the jungle requires an engaged, forward-looking, and, alas, expensive foreign policy. If, for example, you want to put the world on the road to abolishing nuclear weapons, you have to make sure that nonnuclear states like Ukraine don’t have to worry about land-grabs from nuke-wielding neighbors like Russia.

When Ukraine agreed to give up its “legacy” nuclear weapons—missiles and warheads placed on Ukrainian territory when it was part of the Soviet Union—the U.S., U.K. and Russia pledged to protect its territorial integrity. That promise is clearly a dead letter, and it just became much harder to persuade countries that beautifully phrased treaties signed by great powers can replace nuclear weapons as instruments of self-defense.