After Britain voted to leave the European Union,Donald Trump explained, “People want to take their country back. They want to have independence in a sense. You see it with Europe, all over Europe.”

I’ve been traveling in Europe this week, on my way to a conference in Poland. There is resurgent nationalism in some countries. Anti-immigrant sentiment exists. A few right-wing parties are gaining strength.

But European society is deeply cosmopolitan. In the streets of Munich, Bavarians in lederhosen mix with Turks in headscarves. African men in flowing white robes stand on subway platforms with German soccer fans. American brands – McDonald’s, Pizza Hut, Starbucks – compete with German beer gardens, sushi restaurants and falafel stands.

The same is true in the U.S. We enjoy German sports cars, Indian cuisine and Mexican labor. And we profit from the global economy. The Apple store in Munich is around the corner from the cathedral and old city hall.

And yet, politicians appeal to nationalistic nostalgia. In a recent speech supporting Trump, Sarah Palin said, “We’re going to take our country back, and you are either with us or against us.”

But what would we go back to? Contemporary nation-states are modern inventions with a complex and often ugly history. And nationalism creates strife. Each nationalistic line in the sand comes at the expense of someone else.

This is obvious in central Europe. Poland, Lithuania, Hungary and other nations have come and gone, swept along by the tides of history. Some countries have disappeared entirely.

The Kingdom of Bohemia was swallowed up by the Austrian Empire. In the 20th century, Bohemia became Czechoslovakia. Then the Nazis invaded. And the Soviets. After the Cold War, Czechoslovakia broke in two. The Czech Republic was formed in 1993. The country recently adopted a new name, Czechia.

North American history is also full of conflicting identities. Europeans stole the continent from native peoples. The United States was formed out of English, French, Spanish and Russian lands.

Alaska was purchased from Russia. It joined the union in 1959 along with Hawaii. What would we say if native Hawaiians wanted to take their country back?

Some people long for the good old days of homogeneous national identity and nationalistic pride. But there never were such good old days. Nostalgia is a poor guide for the present because it is not based in reality. The truth is that political life is constantly changing. Ethnic, racial, linguistic and cultural identities are fleeting realities.

The dangers of nationalistic identity are obvious in central Europe. Concentration camps dot the landscape, along with memorials for those killed in the nationalistic wars of past centuries. Here in Munich, Hitler began the process of transforming Romantic German myths into the monstrous deeds of the Third Reich.

Nostalgia can be fun. My American passport contains inspiring scenes from the American past. There is a picture of a pioneer plowing with oxen and another picture of cowboys herding cattle. But no modern farmer would trade his tractor for a team of oxen. And it’s no longer common that cowboys ride the open range.

The past was often worse than the present. And it was at least as complicated. The reality of the past included dysentery, racism, war and the subordination of women.

There is no going back. We can only move forward. We are in the middle of an unprecedented economic, cultural, technological and ideological transition. We are moving away from isolated nations toward a global society based upon shared notions of human rights.

Not everyone likes that development. But the genie of globalization is out of the bottle. Most of us benefit from it. Open travel, free trade and cultural admixtures make life profitable and interesting.

The Czechs, Poles and Germans seem to have discovered a way to coexist. Despite the atrocities of the nationalistic past, they are participating in the cosmopolitan European experiment.

We ought to embrace the cosmopolitan future, while defending human rights and a global ethic of compassion. That requires an educated understanding of history. Superficial slogans mislead us. Nostalgic myths are dangerous and divisive. The task of the future is to discover and celebrate our common humanity.