The first Iraq War was one of necessity because vital U.S. interests were at stake, and we reached the point where no other national-security instruments were likely to achieve the necessary goal, which was the reversal of Saddam Hussein's invasion and occupation of Kuwait.

There's a pattern in Bush 43's presidency of being attracted to the big and the bold, and my whole reading of him is that he was instinctively uncomfortable with what you might call a modulated foreign policy - a foreign policy of adjustment, of degree.

When great powers fade, as they inevitably must, it's normally for one of two reasons. Some powers exhaust themselves through overreach abroad, underinvestment at home, or a mixture of the two. This was the case for the Soviet Union. Other powers lose their privileged position with the emergence of new, stronger powers.

Donald Trump's United States is not isolationist. He has authorized the use of limited military force against the Syrian government in a manner his predecessor rejected.

Trump is the first post-World War II American president to view the burdens of world leadership as outweighing the benefits.

Trade accords had been a staple of the post-World War II world, providing a mechanism for economic growth, development, and association with friends and allies, and a means of reining in would-be adversaries who otherwise would have little incentive to act with restraint.

What is obvious is that Donald Trump is comfortable with an approach to running his presidency based on what worked for him in the private sector.

Any time you use military force, you have got to have a clear purpose that military forces can achieve.

I tend to be one of those who does not equate democratization with the holding of elections. The emphasis ought to be on such things as rule of law, economic reform, and promotion of a free media - in short, essentially independent, free institutions.

Vietnam was not a war of choice the United States had to fight. It was clearly not central.

For President Bush, the first, the 41st president, George Herbert Walker Bush, I spent all 4 years of his presidency on the staff for the National Security Council.

There is no way to know for certain what accounts for North Korean decisionmaking, given how closed a country it is.

No amount of sanctioning will persuade North Korea to give up nuclear weapons, nor will China step up and solve the problem for us.

How can we pressure China on North Korea if China's one of the two largest holders of American debt?

Our inability to govern ourselves at home, to deal with everything from infrastructure to our debt to tax policy, is reducing the appeal of the American model.

I did not believe in the Iraq war.

Campaigning and governing are two very different activities, and there is no reason to assume that how Trump conducted the former will dictate how he approaches the latter.

If you want to encourage some activity, make it easy.

Behavioral economics offers a plausible explanation for overreactions by the market. For example, a long period of bad performance can lead to stereotyping.

Whenever I'm asked to autograph a copy of 'Nudge,' the book I wrote with Cass Sunstein, the Harvard law professor, I sign it, 'Nudge for good.' Unfortunately, that is meant as a plea, not an expectation.

In the 1940s, economics started getting highly mathematical. It was basically because economists weren't smart enough to write down models of real behavior that they started writing down models of highly rational behavior - and they kind of forgot about humans.

The lesson of my field, behavioral economics, is that we need to understand the ways in which we differ from the rational human assumed in standard economic theory.

We all need a lot of humility, and especially about the economy.

Countries all around the world, starting with the U.K., have started behavioural insight teams, often referred to as nudge units. And they seem to be doing lots of good.

If you're not putting enough away for emergencies or retirement, making commitments in advance, such as signing up for payroll withholding, can help.

Real people have trouble balancing their checkbooks, much less calculating how much they need to save for retirement; they sometimes binge on food, drink, or high-definition televisions. They are more like Homer Simpson than Mr. Spock.

Arthur Laffer's idea, that lowering taxes could increase revenues, was logically correct. If tax rates are high enough, then people will go to such lengths to avoid them that cutting taxes can increase revenues. What he was wrong about was in thinking that income tax rates were already so high in the 1970s that cutting them would raise revenues.

Lotteries are just one way to provide positive reinforcement. Their power comes from the fact that the chance of winning the prize is overvalued.

I am all for trying to teach household finance in schools, starting as early as possible. And when it comes to high school, I think learning about compound interest is at least as important as trigonometry or memorizing the names of all 50 state capitals.

Leaders are important but not omnipotent.

If there is one thing that most economists agree about in the realm of tax policy, it is that it's best to broaden the base of any tax, all else being equal. That means minimizing the number of deductions and exclusions from taxable income in order to lower marginal rates and reduce distortions.

The sad truth is that many behavioral economists know very little about psychology.

The more we turn down questionable offers like trip insurance and scrutinize 'one month' trials, the less incentive companies will have to use such schemes.

Pundits are no better at forecasting election outcomes than they would be at predicting the final path of a hurricane. Smart pundits should consider either abandoning this activity or consulting with the geeks before rendering their guesses.

If governments want to encourage good citizenship, they should try making the desired behavior more fun.

It's not that we can predict bubbles - if we could, we would be rich. But we can certainly have a bubble warning system.

The main thing that you learn in grad school, or should learn, is how to think like an economist. The rest is just math.

One reason for high health care costs is that patients fail to follow their treatment regimen.

Signing up to be an organ donor should be at least as easy as downloading a song to your iPhone.

I try to teach people to make fewer mistakes. But in designing economic policies, we need to take full account of the fact that people are busy, they're absent minded, they're lazy, and that we should try to make things as easy for them as possible.

It's essential that we understand things like the free-rider problem, but we also need to understand that, fortunately, humans are a little nicer than economists give them credit for. Some people actually leave money at roadside fruit stands; some people give money to NPR so we can listen to it.

Tax cuts are one of many ways to stimulate the economy. Building infrastructure, for example, is another.

The tradition of Chicago price theory is a good one, and it is a low-tech methodology that tries to apply simple economic theory to the world.

If we think that high marginal tax rates are bad because they distort incentives, the same is then true for tax subsidies.

Parents want their children to excel, callers to a victims' hot line want help, and sick people want to get well. Offering aids is like providing an alarm clock: it may help people get to an appointment on time, but no one is forcing them to use it.

One of society's thorniest problems is that children from poor families start school lagging badly behind their more affluent classmates in readiness.

The lesson for businesses is you are dealing with real people. Those are your customers, those are your employees, those are your bosses, and the better you understand how real people tick, the more successfully you will be able to accomplish your goals.

The good thing I will say about the Chicago School is that it was always about the world, not about the abstract.

How can government reduce the frequency and the severity of future catastrophes? Companies that have the potential to create significant harm must be required to pay for the costs they inflict, either before or after the fact. Economists agree on this general approach. The problem is in putting such a policy into effect.

When it comes to assessing the chances of some complicated combination of events, gut feelings are pretty much useless.