But this notion is difficult to apply when confronting the conflicts arising over monetary union and immigration. The massive challenge of dealing with millions of migrants from Asia and Africa has tested the very concept of unity. In the absence of a common policy, geography is decisive, and the burden falls heavily on the union members least able to bear it. Fears over immigration contributed to the Brexit vote and have led to continuing unease among other member states. At the same time, however, free movement of labor is important to economic growth in Europe, as it is in the United States.

The fiscal crisis in Greece and the flood of migrants into Italy and other European Union countries have thrust this debate on the Europeans, and they must continue the debate, even intensify it. They must decide on their objective and then adjust their organization and its institutions and powers to that goal. A continuing mismatch between ends and means will complicate the union’s future, and may even threaten its existence.

The United States has a huge stake in the outcome. In little more than a year in office, President Trump unwisely has stalled a trade agreement with the European nations and has withdrawn from the Paris climate accord, the agreement with Iran on its nuclear program and the Trans-Pacific Partnership. In addition, the president has sought to resolve trade disputes with China and European countries through tariffs, rather than through the trade-dispute mechanisms of the World Trade Organization.

His recent trip to Europe was confusing, contentious and counterproductive, as he continued to criticize our friends and praise President Vladimir Putin of Russia. I respectfully but strongly disagree with Mr. Trump. Cooperative efforts with our historic allies are not harmful to American interests. To the contrary, these recent agreements and the post-World War II institutions have been beneficial to those who participated in them, including and especially the United States. Any American who thinks the world is unsafe now should contemplate a world in which there is no NATO, no European Union, no World Trade Organization, no U.N. In that world, constant trade wars could lead to real wars, and the United States, as the dominant power, invariably would be called upon to lead alone.

Our ties with Europe predate the establishment of our country. We gained our independence from England by revolution, but we retained England’s language, the spirit of its laws and many of its customs. Although our early relations were hostile, over time the two countries formed what remains a “special relationship.”

As our nation grew to settle a vast continent, we welcomed millions of immigrants from England, Ireland, Germany, France, Italy, Greece, Poland, Scandinavia and many more. As a result, we share deep bonds of blood with Europe, not just legal relationships.

While we compete in many ways, we should not think of Europeans primarily as adversaries. They also are our partners and our allies. Although they do not always agree with us, or even among themselves, for the most part they admire our country and share our values and interests. It is in everyone’s interest that we do all we can — politically, economically, militarily and otherwise — to help the people of Europe remain democratic, united, free and prosperous.