William F. B. O'Reilly Portrait of Newsday/amNY columnist Bill O'Reilly (March 28,

William F. B. O'Reilly is a consultant to Republicans.

I went to Europe last week, ostensibly to celebrate a friend’s 50th birthday. What I really was there to see — and feel — were the reactions of Europeans to mass Muslim immigration from Syria and elsewhere. From some news reports, one would think that panic was sweeping the continent. If that’s the case, I didn’t sense it. Not in Paris, not in Amsterdam — not even in Cologne, where hundreds of German women reported being sexually assaulted on New Year’s Eve by mobs of north African immigrants.

There were lots of cops walking around with intimidating weapons, but other than that, the cities appeared calm and strikingly more racially homogeneous than, say, New York City.

I did have a notable conversation with a leftist middle-aged businessman in Amsterdam who tried to rationalize what happened in Cologne. (It’s understandable that male Middle Eastern refugees might sexually attack women, he said. They’ve never seen women uncovered in public and they know punishments in the European Union are comparatively minor.)

But when I asked how long he thought it would take for the immigrants to fully assimilate — to become Dutch, French or German first — he laughed, saying that’s a singularly American concept.

The rise of billionaire Donald Trump as the leading Republican candidate for president is the best evidence of the fear gripping America when it comes to illegal immigration. Lots of us, including yours truly, are genuinely afraid that America is changing too rapidly, that it’s getting away from us.

But the Amsterdam leftist reminded me of what’s truly exceptional about our country: We can come from anywhere and become Americans above all in a single generation.

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My Republican brethren have lambasted President Barack Obama effusively for his reluctance to criticize Muslims writ large for the evil acts of a few fanatics. But I find Obama’s strategy more than sound. He seems to understand that Islamic terrorists want nothing more than division between Muslims and non-Muslims in America, thus impeding the classic American assimilation process. Allowing that division to happen would be a tragic mistake and must be avoided at all costs.

The United States undeniably has work to do in securing its borders. No nation can maintain its sovereignty if its borders are a sieve. But perhaps the more important conversation we should have is how to ensure that traditional Americanization continues. Because if immigrants become Americanized, who cares where they originated?

That conversation requires outlining what it actually means to be an American. And it requires teaching newcomers about shared values and abandonment of old-world grudges.

If we were doing a better job of that, I suspect there would be a lot more optimism and a lot less fear going around.

William F. B. O’Reilly is a Republican consultant.