No, the terrorist attack in Pittsburgh is not “incomprehensible.”

I write from the suburbs of Chicago, where I’m visiting for the weekend – not far from Skokie where, forty years ago, a band of Illinois Nazis sought to march in full regalia. Why Skokie? Because in the 1970s it was not only densely Jewish, but also because it had the highest concentration of Holocaust survivors of any other municipality in America. Sticking their hate in the faces of Shoah victims was a tactic for noxious, evil people to most provocatively display their message—one that keeps surfacing since the 2016 political campaign, and Charlottesville, and now Pittsburgh: “You (Jew) will not replace us.”

The massacre of Jews at prayer at the Tree of Life synagogue in Pittsburgh on Shabbat morning was first and foremost a crime against Jews: the deadliest antisemitic attack in American history. Victims do not appreciate having crimes against them universalized. This attack was specifically against Jews, in a Jewish place, marking a moment in Jewish time (Shabbat; and the bris celebrating a baby boy’s arrival into the covenant of the Jewish people).

It is crucial to understand that antisemitism is not “generic bigotry.” It is specifically anti-Jewish hatred, incubated throughout the centuries and always ready to take root in the fertile soil of the far left and the far right.

In the taxonomy of hate, antisemitism has specific characteristics. Similarly, Islamophobia has its own unique expressions, and Muslims’ experience of bigotry is uniquely their own. So, too, for anti-black racism. And homophobia. And all the other special hatreds that the human soul has devised for itself.

However, there is a line that connects modern American hate together, and that line is white supremacy, which has plagued this country from its founding to today.

It’s a thread that runs from the days when Americans owned people of a certain color skin. It was enshrined in a Constitution that considered such a man 3/5 of a human being. It is self-evident on the slobbering faces of white celebrants at lynchings.

It was there when an antisemitic mob murdered Leo Frank in 1915. It runs through the internment camps in which Japanese-Americans were imprisoned during World War II. It was on the MS St. Louis which was turned away from Florida’s shores, bringing its doomed passengers back across the Atlantic to the clutches of the Nazis. It lingers in Quran-burnings by hypocritical preachers, and in vandalized mosques.

It was there in Skokie, and in the massacre at the Pulse nightclub in Miami. And it’s there in the denigration of refugees as something less-than-human.

The perpetrator of the Tree of Life slaughter made his motivations perfectly clear (no, the crime is not “incomprehensible”). He despised Jews in general, and in particular for their perceived role in protecting refugees from seeking sanctuary in America. He called out HIAS (formerly the Hebrew Immigrant Aid Society), and claimed a last straw to save America from invading armies of dark-colored immigrants, as manipulated by sinister Jewish forces.

He told us why. It’s not incomprehensible. Just evil.

White supremacy, white nationalism, whatever you want to call it: it’s the moral rot eating at American democracy since the beginning.

The only peace I can find is that another parallel line likewise runs through the American soul. From the unique experience of a specific group, we can come to partially and incompletely come to understand the suffering (and, I hope, the aspirations and joys) of another group. This is empathy, the greatest of human virtues. Occasionally we confront fellow humans who are completely lacking in this trait. But the gatherings and the vigils of the past few days tell me that it’s possible, at least, that a coalition of decency can arise.

Jonathan Greenblatt said it quite eloquently: You have to have zero tolerance for this.

If your candidate is attacking George Soros or the “globalists,” or a member of Congress from your party is embracing Holocaust deniers, you must stand up and tell them to stop.

If your allies in a range of social justice causes either explain away the anti-Semitism of the Nation of Islam by citing the good work they may do or justify demonizing the Jewish state of Israel and its existence, then they need to know that they can no longer be your ally.

If your favorite social media platform continues to refuse to remove anti-Semitic garbage from its site, then vote with your clicks and deactivate your account.

When we consider this horror in the days and weeks to come, we should keep that in mind. It is about the poisonous sprout of white supremacy – and those who would enable it with their silent nods and coded dog whistles.