That was a telling justification that President Obama gave last week for his decision to kill the Keystone pipeline that could have created 42,000 temporary jobs and increased America’s independence from oil produced in such problematic countries as Nigeria, Venezuela, and Saudi Arabia.

A State Department review found little impact on the environment from the pipeline, which could actually result in a net emissions decrease because it is a cleaner way to move oil than via railroad and truck. Obama had to concede that Keystone would not be an “express lane to climate disaster” predicted by green groups. So why, then, did he reject Keystone — aside from his obvious desire to cater to an important Democratic Party constituency? “America’s now a global leader when it comes to taking serious action to fight climate change,” Obama said. “And frankly, approving this project would have undercut that global leadership.”

In short, Obama’s decision on Keystone was driven by the hoary old theory that the U.S. needs to model good behavior for other countries in the hope that they will emulate what we do. It is not only on the environment that Obama has embraced this theory. So, too, on nuclear weapons where he has embraced the goal of “nuclear zero,” negotiated a New Start arms-reduction treaty with Russia, and cut spending on America’s nuclear deterrent in the hope that if we cut back our arsenal other nations will follow suit. And on the treatment of terrorist detainees, Obama has banned all coercive interrogations and is going to make another attempt to close the Guantanamo Bay detention facility before he leaves office.

This is part of a long tradition which holds that America needs to be a “Shining City on the Hill” that inspires the rest of humanity, and there is some obvious validity to this: America’s freedom and prosperity continue to inspire envy and emulation in the rest of the world, and we are better off if other nations adopt liberal democratic norms. But there is scant indication that when America voluntarily weakens itself — whether on energy, nuclear deterrence, fighting terrorism or any other security-related realm — other nations, especially hostile nations, will weaken themselves, too.

All you have to do is recall the 1920s when the Harding and Coolidge administrations, in a fit of isolationist idealism, signed the Kellogg-Briand Pact banning war as an instrument of state policy and the Washington Naval Treaty limiting naval armaments, while at the same time reducing the U.S. armed forces to the level of an international laughing stock. Those treaties did nothing to slow down the rearmament of Japan and Germany, nor did they make World War II less likely — quite the opposite, they hindered the Western democracies in responding to totalitarian aggression and encouraged Hitler and Tojo to become more aggressive. (Why some on the right are eager to rehabilitate Harding and Coolidge, given their dismal record on national security, I have no idea — but that’s a subject for another day.)

The logic of unilateral concessions works no better today than it did in the 1920s. ...