Antisemitism is as old as the Jewish story itself. As Jews, we’ve lived with the threat of radical groups trying to eradicate our people for centuries. From Egypt to Poland to Persia, the Jews have experienced unbelievable persecution. Today in the 21st century, antisemitism is still alive and growing, even here in America.Despite years of persecution, a few things have always kept us strong: our beliefs, our values, our culture, our history – and our homeland, Israel.History has shown that antisemitism does not discriminate. As novelist Norman Mailer once said, “When the time comes, they won’t ask what kind of a Jew you are.” Nobody asked the congregants at the Tree of Life synagogue in Pittsburgh or at the Chabad of Poway.It doesn’t matter if you go to synagogue every day, every week, once a year, or you haven’t been since you were a child. It doesn’t matter whether you affiliate with one political party or another. And it doesn’t matter if you consider yourself an American Jew, or an American who just happens to be Jewish.When we see antisemites acting out their hateful bigotry – as we’ve seen from Pittsburgh to Poway to the halls of Congress – we are confronted with a choice: do we condemn the evil and hope for the best? Do we turn our backs on the future of our people and allow history to repeat itself? Or do we fight back?For me, the choice is clear.Today’s antisemitism manifests as a three-headed monster that spews its vile bigotry through the radical Right, the radical Left, and radical Islam. Each one of these hateful heads poses a real threat not only to Jewish people and to Israel, but also to the US and the Western world.In recent years, North America has joined Europe in witnessing a growing alliance between radical Muslims and radical leftists. Radical Muslims stone women and detest feminism, execute gays, systemically oppress minorities, and disregard basic human rights. On paper, the radical left should be appalled by this ideology, but these unlikely allies turn a blind eye to the tenants of radical Islam because they share common hatred for Western influence in the world, pluralistic nationalism, and Jewish power.Just as the radical Left has a history of vilifying Israel, the nation-state of the Jewish People (and the Jews who support it), the radical Right has colluded with radical Muslims to spread its pure hatred of Jews and the desire to brutally subjugate – or murder – the Jewish people.This isn’t the first time. Grand Mufti of Jerusalem al-Husseini conspired with Hitler less than 80 years ago. Today, the rhetoric of neo-Nazis and radical Muslims echo one another as both groups continue to spew hateful conspiracy theories and centuries-old antisemitic tropes.The only way to defeat this monster is to put aside our internal political differences and band together as proud, courageous Jews.But how? The struggle is two-fold.First, we must learn all we can about those who seek to destroy us. That means investing in research capabilities and drawing on the latest technology to uncover the antisemitic networks online and off. Then, we must use this knowledge to expose their unlawful activities and better protect our Jewish institutions in partnership with law enforcement.Second, we must fight the alarming encroachment of antisemitism from the extreme fringes of our politics into the mainstream. We must be vigilant, calling out antisemitism wherever we see it. We must also educate the public about the dangerous and insidious nature of seemingly minor antisemitic comments. We must call out hate wherever we hear it, whether its spewed by Louis Farrakhan, Rep. Rashida Tlaib or David Duke.For too long we have behaved as if antisemitism is merely distasteful, but not a danger.The recent deadly attacks on synagogues in Pittsburgh and Poway make the stakes clear for us all. Anyone who exhibits or espouses antisemitism must be immediately called out and held accountable.Jews don’t have to be like sheep led to the slaughter. Unlike Jewish communities in the past, we have human and civil rights, and the ability to fight back.We are a proud people with a long memory, and lucky to live in a time with a strong and thriving Jewish state. We no longer have to be afraid and passive. We must switch from defense to offense. To defeat antisemitism, we must be proactive and fight this head on. If we don’t, history shows that the results could be catastrophic.Adam Milstein is an Israeli-American real estate investor and philanthropist. He and his wife, Gila, founded the Adam and Gila Milstein Family Foundation, and he is a co-founder of the Israeli-American Council, where he serves as chairman of the board.