If the current violence between Israel and the Hamas terrorist regime in Gaza escalates into a full-scale war, one thing is certain. The main thoroughfares of the West’s great cities will be filled with thousands of protesters marching in support for Hamas and its strategic goal of annihilating Israel.The anti-Israel demonstrations this time around will dwarf all those that preceded them.We also know with mathematical certainty that Jewish institutions and Jews will be violently assaulted from London to Melbourne, Paris to San Francisco.Every public demonstration in support of Israel, indeed, every public demonstration of Judaism, including Jewish people walking to synagogue on Shabbat – will become a focal point for attacks.We also know with absolute certainly that just as the violent assaults against Jews in the West during Operation Protective Edge in 2014 dwarfed the attacks from earlier wars, so the violence directed against Jews and Jewish institutions in Europe and the US in the next war will dwarf all that preceded it.We know that the anti-Israel demonstrations will be bigger and violence against Jews more widespread now than in the past because over the past four years, leftist antisemitism has grown in scope and extremism throughout the Western world.Hatred of Jews, based in a rejection of Israel’s right to exist and expressed first and foremost through the demonization of Israel’s supporters as racists, has become more widespread. Its expressions have become more extreme and more violent.Consider the situation in Britain. Last month, US President Donald Trump paid a visit to the US’s closest ally. The Red-Green alliance of the Left and the Muslims was beside itself. Its queer, feminist, jihadist and animal rights members banded together to organize a major demonstration against Trump in London.As British Jewish writer and activist David Collier documented on his website, two aspects of the demonstrations stood out. First the marchers weren’t against Trump. They were anti-American.As far as they were concerned, Trump is just a manifestation of America’s inherent evilness. His predecessor Barack Obama was also terrible.The second notable aspect of the supposedly anti-Trump, Red-Green rally in London was the prevalence of anti-Israel messages. Collier posted videos of crowds at every corner happily shouting out chants calling for Israel to be destroyed, (“From the River to the Sea, Palestine will be Free,”) and supporting Hamas, (“From Palestine to Mexico, racist walls have got to go!”).It’s worth asking how the protesters at an antiTrump rally naturally gravitated towards anti-American and antisemitic messages.Why did the people who supposedly hate Trump snap up “Free Gaza” stickers like hotcakes? What possesses British feminists to support a jihadist regime that routinely attacks Israel for no reason and treats women like property? The answer is simple enough. They aren’t thinking. They are following.And over the past 15 to 20 years, hatred of Israel and support for its enemies, including Iran, Hamas and Hezbollah, has moved from the margins of the Left to become its signature foreign policy. What was once the cause célèbre of the Left’s radical fringes has now become the default position of politicians and activists associated with the Left throughout the Western world.Take for example Sen. Cory Booker’s recent participation at the annual Netroots Nation conference, which took place last weekend in New Orleans. The conference, first convened during Barack Obama’s presidency, bills itself as “a political convention for American progressive political activists.” It has become a key tool for fundraising and political organizing for Democratic politicians. Would-be presidential candidates cannot afford to miss it.Given its “progressive” pedigree, the only foreign policy position the participating groups advocated on behalf of was abandoning US support for Israel and replacing that support with support for the Palestinians. To this end, the US Campaign for Palestinian Rights, (USCPR) was a major participant in the conference.Booker’s political roots are in the pro-Israel community.And until Obama concluded the nuclear deal with Iran three years ago, Booker was one of the most outspoken supporters of Israel in the Democratic party.Booker smiled broadly in the picture he took with the USCPR members. In the photo, Booker happily clutches a sign which reads, “From Palestine to Mexico, all the walls have got to go.”When asked about the photo by reporters, Booker’s spokesman insisted that Booker hadn’t read the sign.But USCPR activists dispute his position. They told The Intercept that they spoke to him before snapping the photo. One of the members in the photo is wearing a t-shirt that says, “Palestine is a queer, feminist, refugee, racial justice issue.”If he didn’t read the sign, he should have been able to guess it wasn’t an ad for a kosher deli.THIS BRINGS us back to Britain.Two weeks ago, in an unprecedented move, Britain’s three national Jewish newspapers closed ranks and jointly published the same front-page editorial.The editorial dealt with the growing prospect of a Labour government under the leadership of Labour Party chairman Jeremy Corbyn. The papers referred to that possibility as “an existential threat to Jewish life in this country.”According to the joint editorial, “With the government in Brexit disarray, there is a clear and present danger that a man with a default blindness to the Jewish community’s fears, a man who has a problem seeing that hateful rhetoric aimed at Israel can easily step into antisemitism, could be our next prime minister.”The Jewish newspapers were driven to publish the joint warning by the Labour Party’s refusal to adopt the same definition of antisemitism as the British government, 130 local governments and dozens of other countries have adopted. The definition, which was drafted by the International Holocaust Remembrance Alliance, (IHRA) enumerates eleven examples of antisemitism to enable authorities to properly identify anti-Jewish discrimination.Included in the examples is the allegation that Israel is inherently racist and comparisons of Israel to Nazi Germany. Labour based its rejection of the IHRA definition on the presence of those examples.As far as Corbyn’s Labour party is concerned, it is not necessarily antisemitic to argue that Israel is inherently racist and therefore has no right to exist or to compare Israel to Nazi Germany.Labour’s refusal to accept IHRA’s definition on these grounds is not merely a testament to the fact that the view that Israel the illegitimate, modern incarnation of Nazi Germany is commonplace in the party today. Indeed, Corbyn and his close advisors have been repeatedly documented voicing such views.It is also a declaration of intent for the future.By refusing to say that it is antisemitic to claim that Israel is inherently racist, Labour is justifying, and to a degree, inviting open discrimination and persecution of Jews.If Corbyn becomes Britain’s prime minister, Labour’s position will open the door to institutional discrimination against Jews. Due to their Zionism – that is, their “racism” – British Jews are liable to find themselves boycotted, fired, socially, academically and professionally shunned and sued. After all, if they are Zionists, they are evil.British Jews aren’t the only ones who need to be alarmed by Corbyn’s rise, and the Left’s ever-growing embrace of antisemitism. Israel also needs to be deeply concerned.Not long ago, Corbyn referred to terrorists from Hamas and Hezbollah as “my friends.” If he becomes British prime minister, not only can Israel expect his government to suspend all security cooperation. Israel can reasonably expect Britain to start funding – and transferring strategically significant information – to Corbyn’s “friends.”And that’s just the tip of the iceberg. The main threat Corbyn poses to Israel stems from Britain’s alliance with the US.In the century that preceded Trump’s presidency, the British used their close relationship with Washington to diminish US support for Israel. In the immediate aftermath of the September 11 attacks for instance, then British prime minister Tony Blair convinced then-president George W. Bush to limit the scope of his anti-terror campaign to terror groups that didn’t primarily attack Israeli Jews. Due directly to Blair’s intervention, in his September 20, 2001 speech before the joint houses of Congress, Bush declared war on terror organizations “with global reach.”Thanks to Blair, given the “local” nature of their terror war against Israel, Palestinian terrorists received ever expanding political, military and financial support from the Bush administration.Indeed, two months after Sept. 11, then secretary of state Colin Powell became the first senior US official to officially express support for Palestinian statehood.IF CORBYN rises to power while Trump is still in the White House, he will speed up the ongoing radicalization and legitimize the antisemitic trajectory of the Democratic Party. But his immediate effect on US policy towards Israel will be limited.If, on the other hand, Corbyn serves in parallel to a Democratic president, given the radicalization of the Democratic Party, a Corbyn government will have a poisonous effect on US-Israel relations.In light of the stakes, Israel needs to work on three levels simultaneously.First, Israel needs to fight for British Jewry. The Foreign Ministry, and national institutions like the United Israel Appeal, and even regular Israelis on vacation in Britain – need to be as outspoken in criticism of Corbyn as the Jews of Britain are.There is no point in being diplomatic. If Corbyn forms a government, diplomatic niceties will get us nowhere.British Jews now courageously fighting Corbyn with everything they have are not being alarmist.As the three papers noted, until recently, the Labour Party “was the natural home for our community,” but “Corbynite contempt for Jews and Israel,” has eroded “its values and integrity.”Second, Israel needs to prepare for the day after Corbyn wins the next general election. Any longterm strategic projects Israel may have with Britain should be wound down now. New projects should not be initiated.Israel-based anti-Israel NGOs with ties to the Labour party should be followed very, very closely.As far as the US is concerned, given the ongoing radicalization of the Democratic party and its ties with Britain’s Labour Party, Israel’s continued strategic dependence on US military aid is looking less and less responsible.Israel should use Trump’s tenure in office as an opportunity to transform our military dependence into a military partnership. We should exchange aid for US investments in joint weapons research and development projects. By transforming our relationship into a partnership, Israel will minimize the chance that the next Democratic president will feel comfortable walking away from Jerusalem in lockstep the party’s radical grassroots and his or her British counterpart Corbyn.The growth and escalation of antisemitism in the Western Left is a strategic threat to both Israel and the Jewish communities in Western nations. For the future of Western Jewry and of Israel, we need to recognize the threat and take action to limit its consequences.