Just as radar warns of approaching storms, so does the flood of migrants entering Europe warn us of a deluge yet to come, not only for Europeans, if they continue to allow unrestricted immigration, but for the United States.

Reports that women in Cologne, Germany, have been groped and robbed by men described by authorities as having "a North African or Arabic" appearance should be warning enough, but there are other and more ominous warnings that suggest worse lies ahead, unless the problem receives immediate attention and action. And it's not just Cologne.

The Gatestone Institute, a nonpartisan, not-for-profit international policy council and think tank, is in possession of what it describes as a "leaked German intelligence document," which says, "We are importing Islamic extremism, Arab anti-Semitism, national and ethnic conflicts of other peoples, as well as a different understanding of society and law."

Last October, reports Gatestone, Andrew Parker, the director general of Britain's Security Service, said that "'the scale and tempo' of the danger to the UK is now at a level he has not seen in his 32-year career. British police are monitoring over 3,000 homegrown Islamist extremists willing to carry out attacks on the UK."

On Wednesday, President Obama visited a Baltimore mosque. According to The Daily Caller, the mosque "has deep ties to extremist elements, including the Muslim Brotherhood." That mosque is not alone, as a map on the paper's website reveals.

Explaining the president's visit, White House spokesman Keith Maley said, "The president believes that one of our nation's greatest strengths is our rich diversity."

I doubt terrorists believe that. I don't believe that diversity, as practiced in America, exists in any country with a Muslim majority.

Benedicte Bjornland, head of the Norwegian Police Security Service, recently warned against further Muslim immigration. When U.S. politicians suggest a similar approach, they are denounced as "bigots" and "Islamophobes," but in Norway and Sweden, two of the most liberal nations in Europe that have welcomed Muslim immigrants, that charge will be difficult to make stick.

What we are witnessing is the complete breakdown and failure of multiculturalism. Dictionary.com defines multiculturalism as "the preservation of different cultures or cultural identities within a unified society, as a state or nation."

That definition contains a glaring contradiction. A society cannot be unified if it preserves different cultures and cultural identities within itself. That's why our national motto is translated "out of many, one." To the multiculturalist it appears to be, "Out of one, many."

History demonstrates that no nation can long survive if it forgets why it exists. Our failure to inculcate American traditions, beliefs and history, even in the native born, not to mention immigrants, is rapidly destroying the country bequeathed to us by our forebears.

Leftists in Europe and the U.S. have promoted multiculturalism, believing that once Muslims experience our freedoms and dedication to equality they will want to be like us. It doesn't appear to be working and anyone familiar with the Koran and its "kingdom of this world" instructions knows it likely won't.

European leaders, from Germany's Angela Merkel, to Swedish Prime Minister Stefan Lofven, have deliberately closed their eyes to what they see unfolding in their countries, and in others.

President Obama is doing the same thing with his trip to the Baltimore mosque. Our enemies see our weakness and failure to understand their objectives, which include destroying the West and establishing a worldwide caliphate. This is not top secret information. Not all Muslims are terrorists, to be sure, but large numbers of radical Islamists profess allegiance to the faith and they are more than willing to wreak havoc in pursuit of their goals.

An ancient proverb reminds us: "There are none so blind as those who will not see."

Cal Thomas is America's most widely syndicated op-ed columnist. He joined Fox News Channel in 1997 as a political contributor. His latest book is "What Works: Common Sense Solutions for a Stronger America" is available in bookstores now. Readers may email Cal Thomas at tcaeditors@tribune.com.