SAN DIEGO -- You want to talk about a border emergency?

Grab your passport, and take a quick trip with me around the world. Look at the tensions between Israel and Syria. India and Pakistan. Iraq and Iran. Colombia and Venezuela. South Korea and North Korea.

Now, closer to home, look at this relationship: United States-Mexico.

Thank goodness for good neighbors. Americans really need to stop hyperventilating about the supposed crisis on their southern border -- better known as the natural migration of people from one place to another -- and count their blessings.

I'll start. Americans are fortunate that Mexican law enforcement provides regular intelligence on terrorist threats along the border. In 2011, then-U.S. Attorney General Eric Holder credited Mexican authorities with helping uncover a murder-for-hire plot to assassinate the Saudi ambassador to the United States. The plan came to light when Manssor Arbabsiar, a naturalized U.S. citizen from Iran, met with a person he thought was part of a Mexican drug cartel but who was actually a confidential source for the Drug Enforcement Administration.

You're welcome, America. Or as the Mexicans say: (BEG ITAL)"De nada."(END ITAL)

A good friend is an anti-terrorism hawk who studies the U.S.-Mexico border and who recently gained access to a migrant caravan heading north through Mexico. What keeps him up at night are the so-called "special-interest aliens" from terrorist-producing countries who roam around the border, and what their intentions are.

According to the libertarian Cato Institute, as many as 45,000 special-interest aliens have been captured by U.S. authorities since 2007. And yet there has never been a terrorist attack on U.S. soil linked to this population.

Yet when I speak to groups about immigration, I never hear a peep about potential terrorists. You know what I hear? This:

"We have to stop this invasion of illegal aliens from Mexico who refuse to learn English and assimilate into our culture."

As the Republican-led Senate pushes back against President Trump's attempt to declare a "national emergency" on the U.S.-Mexico border to clear a path for him to build his "big, beautiful wall," it seems like a good time to clarify the exact nature of this emergency.

The real four-alarm fire is a blinding fear of changing demographics and a clash of cultures that has worried nativists and other immigration restrictionists for 30 years. It was in the early 1990s that immigration from Mexico started surging and the media began running stories -- in the march to the 2000 census -- about how the Latino population was swelling and how whites would eventually become a statistical minority.

That freaked out a lot of people. A decade later, some people were even more frightened by those massive immigrant marches -- in cities like Dallas, Phoenix, Chicago and Los Angeles.

Over the years, fear gradually turned to resentment. Now resentment has morphed into panic -- with a dash of opportunism. For those who worry about changing demographics, Trump's national emergency declaration is a convenient vehicle to stop the browning of America.

That's what this whole calamity is all about. And no wall can fix that. This is about pulling up the drawbridge. This is like shutting down Ellis Island 100 years ago because the Italian immigrants who were entering were thought to be of inferior stock.

Maybe Republican lawmakers know this full well, and they don't have the character to admit that there is nothing high-minded or righteous about Trump's emergency declaration. Or perhaps they're so dense that they really do think what's happening on the border is about keeping out opioids, turning back criminals and rescuing children from human traffickers.

Syndicated radio host -- and self-appointed expert on the U.S.-Mexico border -- Rush Limbaugh sounded the alarm last week when he told listeners: "This crisis is transforming the United States into something it wasn't intended to be. There's no question that there is an invasion going on of people who can't be absorbed or assimilated into our culture."

I appreciate the honesty. Limbaugh is half right. It's a crisis all right.

But it's not about turning away undesirables. Every day, south of San Diego, more than 100,000 people cross the border between the United States and Mexico -- either to make money at work, or spend it at play. Not exactly a war zone.

Rather, it's about what America is turning into -- and those who want to turn it back to what it used to be.

What we have on the border is not a security crisis. It's an identity crisis.