As inconceivable as it may seem, the fear of open physical antisemitism of the classic sort has begun to trickle into the hearts of Jews in the Diaspora. We speak not only of dictatorships or Third World countries, but the most upright democracies in which the commitment to human rights and combating racism have been taken for granted, so it would seem, for at least the last three generations.Australia is an old friend of the Jewish people. Jews moved there from South Africa in both the Apartheid and post-Apartheid eras, when the political situation in the latter was unstable. The newcomers joined Jews who had immigrated Down Under before and after World War II. It is a beautiful love story: Jews from Europe and Africa found a warm home in Australia and repaid the country’s generosity by contributing their talents to the development of the remote island continent.Yet about two weeks ago, the local council in a suburb of Sydney voted to not allow the construction of a new synagogue, on the revolting and absurd grounds that it would be a target for terrorists.The entry to the main synagogue in the city is protected as if it were a military base in hostile territory – locked behind two automatic gates that cannot be opened simultaneously. Another synagogue has armed guards stationed in two lines of defense outside the building.But there are no guards outside Sydney’s churches and mosques; no one is threatening them.In Charlottesville, Virginia, the site of the neo-Nazi demonstration that evoked a controversial response from US President Donald Trump, there is a Reform temple, Congregation Beth Israel. Last week, its president, Alan Zimmerman, described a scene in which three hooligans armed with semi-automatic rifles had paraded ostentatiously outside the building during Sabbath services. Had they been so inclined, they could have massacred the worshipers, because the local police refused to provide an officer to stand guard by the door. There have been Nazi rallies there in the past, punctuated by rhythmic chants of “Sieg Heil,” with swastika flags held aloft.At the end of the service, the congregants had to sneak out of the building through a back door. In 2017, the Jews of Charlottesville decided to remove their Torah scrolls from the sanctuary out of a real fear that an arson attack would be committed in the near future. As a further security measure, they canceled the havdala service at the end of Shabbat. In cyberspace, too, hatred of the Jews is spreading like wildfire. The social networks are full of all types of hate speech – sexual, racial, political and so on – but the antisemitic variety is the most prominent of all. The presence of hatred for Jews and Judaism, wherever found – going beyond the “normal” criticism of Israel – is vast and terrifying. Web crawlers who search the Internet can identify the sources of online antisemitism; it turns out that it comes not only from countries whose regimes are hostile to Israel, but also from citizens of Western countries, among which Britain is especially notorious. It is no wonder that more than a third of the Jews of the United Kingdom feel it necessary to conceal their Jewish identity in public.To the simple question “Why?” there was no answer during the Middle Ages or during the Holocaust. Today is no different. There is no valid explanation for the hatred of Jews because they are Jews. The circumstances and arguments change, the regimes and hegemonic cultures come and go. But the antisemitic demon persists and continues to befoul our world.Unlike in times past, however, today we Jews have our own place on the globe – the State of Israel – where we have marshalled many forces: political and diplomatic, media-based and intelligence-related, technological and military. It is incumbent on Israel to deploy them in an active and determined fight against antisemitism wherever it appears. This is one of the most important implications of the fact that we are the nation-state of the Jewish people, and not just a state comprised of its citizens.Yad Vashem is the institution that preserves the memory of the consequences of antisemitism; Masada is the site where we vow, “Never again.” But the whole world must be the arena of the war against antisemitism and the Jewish nation-state must serve as the supreme commander in this universal conflict.The writer is vice president of the Israel Democracy Institute and a professor of law at Bar-Ilan University.