Nations operate in what they perceive to be their self-interest. It’s not always a good thing.

When the Nazis marched into Czechoslovakia and Poland in 1939, it was under the policy of Lebensraum, the claimed need for food that those fertile lands offered. The people of Poland and Czechoslovakia and the Allies disagreed. When nations’ interests clash, differences must be decided diplomatically or war ensues, as happened in Europe in 1939.

It has always been that way and, likely, always will.

But sometimes a nation’s interest serves the world’s. In the wake of World War II, America’s self-interest — guided by our values — benefitted not only ourselves and the nations who had allied with us, but the people of those countries we helped defeat. In a recent history, “Harry & Arthur,” Lawrence Haas, a senior fellow at the American Foreign Policy Council, tells of the remarkable working relationship between newly sworn-in President Harry Truman and Michigan Sen. Arthur Vandenberg, then the leading Republican on the Senate Foreign Relations Committee.

According to Mr. Haas, a former member of Al Gore’s staff, the Marshall Plan, the Truman Doctrine, NATO and the U.N. Charter would have been impossible without the collaborative efforts of the two men. This was bi-partisanship at its best, relying on fundamental American values — to help those in distress, by serving our own interests.

The consequence: the west saw seven decades of economic growth, Germany and Japan became economic powerhouses and the world saw the most rapid eradication of poverty it had ever known.

Things have changed. We have abandoned our magnanimous perch. Our values are on trial. A belief in moral relativism has replaced a sense of national self-confidence that had been driven by moral certitude.

Political extremism has meant that our nation’s self-interest has been subsumed within the wants of party hacks; and immediate self-gratification has replaced the values needed for moral leadership.

Republicans: consider the effects of the war in Iraq?

Democrats: think of the consequences of the attack on Libya, and the rise of ISIS in Iraq and Syria?

Keep in mind, 9/11 was an attack on Western civilization by Islamic jihadists — a fact that has all but disappeared from our collective memories. Non-threatening euphemisms used to describe those terrorists — and others since — have undermined a focus on the awfulness of what they did, and what they are still capable of doing.

As part of the administration’s bid to rid the world of nuclear weapons, John Kerry became the first secretary of state to visit Hiroshima, the site of the first of two atomic bombs dropped on Japan by the United States. Its purpose was to end a war that had killed an estimated 60 million people.

In the guest book, Mr. Kerry wrote: “War must be the last resort – never the first choice.”

Good words, but what he could have added is that it is not weapons that create devastation, it is the people, governments and policies behind them. It is the values and morality of people, reflected in their leaders, that causes nations to act as they do.

Had the Japanese Imperial Army not invaded China, had they not attacked and invaded Singapore, had they not attacked Pearl Harbor there never would have been the devastation of Hiroshima and Nagasaki. Truman’s decision was morally right. Again, it served our interests and the worlds’.

Our founders knew the tyranny of governments managed by fallible men. They understood that power corrupts. They trusted institutions more than individuals, so they added checks and balances. They ensured a rule of law. They realized habitable society requires civility. They knew that tolerance of intolerance is morally wrong.

It doesn’t make one xenophobic to be disgusted by a Muslim culture that enslaves women and which punishes those women who dare expose an arm. We are not prejudiced because we see evil in the beheading of “infidels,” or in those who spread their religion through jihad.

Moral and cultural relativism blind us to evil. It prevents us from acknowledging that cultures can be morally wrong.

Today, as a nation, our values have drowned in a sea of relativism. It is a nicety to bring together the mothers of black youngsters killed by police, as Hillary Clinton did last week. But should we not also address the far greater prevalence of black-on-black crime? Should we not also gather those mothers and wives of the 40-odd policemen killed in the line of duty in 2015?

An absence of moral certitude raises questions. Why is an unborn a “person” when the mother is threatened with the Zika virus, but a fetus when discussed by Planned Parenthood?

With transgenders, we should respect their wishes to be of a sex different from the one with which they were born. But, in the same vein, is it right for them and their supporters to deny us the respect we give them?

In most public buildings, it is either impractical or too expensive to construct two extra sets of bathrooms — one for transgender females and another for transgender males; and then one for women who prefer to pee only with biological females and another for males who prefer to urinate with those who can stand beside them.

To whom should we cater — a vocal minority or a silent majority? Commonsensical preferences do not make one a bigot.

Where national self–interest is most visible is in foreign affairs. But, how have we done in recent years? As the United States, it is in our interest that the number of democracies around the world expand; instead, the number has shrunk. As a nation that has thrived on a market economy, it is in our interest to promote global trade; instead, trade agreements are under attack by members of both parties.

As an exemplar of democratic capitalism, it is in our interest to further the system that has lifted untold millions from poverty; instead we find ourselves pilloried for inequality. As a country that embodies meritocracy, it is in our interest to encourage the talented, foreign-born to immigrate to this country; instead, we have an immigration policy that welcomes illegals and makes it difficult for those who are qualified and educated from entering the country.

As a beacon of hope that symbolizes freedom and democracy, it is in our interest to show the world the spirit of unity, self-reliance, strength and leadership; instead, politics has become toxic – we are divided: people are increasingly reliant on the state; students are encouraged to seek safe places when uncomfortable with words or phrases, spoken or written; overseas we have opted to “lead from behind.”

In all that we do and in how we live, we are best off in a world at peace; instead, we have a newly combative Russia and a provocative China. Their aggression, along with persistent Islamic jihadists’ attacks on western culture, has made the world more dangerous.

What will it take to restore our national self-esteem, that we are a good nation filled with good people, one that exemplifies fairness and equality? We are not perfect, we have some bad people and there are things we can do better, but, tell me, where else would you rather live?