Terrorist violence goes through cycles, like all fads, as ideologies of hate develop, metastasise, evolve and finally ebb in their perverse allure.
In the 1980s, radical, violent Islamism spread around the Middle East and then to the rest of the world, bolstered by the Iranian revolution, the war in Afghanistan and subsequent key triggers such as the 9/11 terrorist attacks in the US.
In the 1960s and 1970s, the dominant rationale was distinctly left-wing, informing western Maoist groups like the Baader–Meinhof Gang, a West German militant outfit, or anti-colonial forces like Palestinian factions or the South African Umkhonto we Sizwe, the armed wing of the African National Congress.
In the 1940s, before the formation of Israel, Jewish nationalists in Palestine like Menachem Begin and Yitzhak Shamir were the original Middle East terrorists.
That followed a wave of fascist violence throughout the western world in the 1930s. In the immediate aftermath of World War I, anarchism was the most common expression of the rage of alienation.
There is nothing new about white nationalism but in recent years it has developed so much momentum as an organising principle for terrorist massacres, especially in the US, that it is starting to displace radical Islamism as the main inspiration for terrorist attacks today, particularly in the West.
Last week's massacre in El Paso, Texas, allegedly aimed at the Hispanic community, joins a raft of other similar recent atrocities conducted by violent young terrorists, who have cited a white nationalist agenda in a series of attacks aimed at minorities, including deadly assaults on a mosque in Quebec in 2017 and a Pittsburgh synagogue in 2018.
Such terrorist brutality requires not only a mindset of extraordinary cruelty but also a powerful narrative of an existential threat that appears, in the eyes of the perpetrators, to justify such mass murder.
The Norwegian white nationalist terrorist Anders Breivik, who massacred 69 people in 2011, appears to have provided the most potent contemporary template, with his notorious manifesto explaining his racist, Nazi and fascist motivations.
Mr Breivik was motivated mainly by hatred of Muslims but his manifesto outlined all the key themes of the "great replacement" theory that has become the driving force behind the current epidemic of white nationalist terrorism.
This theory thrives in far-right forums and is based on the premise that white people and a western identity are under siege from an influx of non-white immigrants. It is no coincidence that the alleged perpetrator of the Christchurch mosque massacres last March titled his credo The Great Replacement.
It is especially striking that white supremacy has shifted from more traditional assertions of white power based on purported superiority to an existential panic about a non-existent crisis, whereby white people in western societies are supposedly being "replaced" by non-whites, such as North African Muslims and black Africans in Europe or Mesoamericans in the US.
It is often accompanied by the preposterous suggestion that this is a Jewish-inspired plot or a conscious trade-off by western leaders for some implausible benefit. It is fuelled by human beings’ natural tendency to form exclusionary identity groups along supposed cultural, ethnic or religious lines.
This particularly paranoid narrative emerged at the beginning of the 20th century in the US through the racial theories of the eugenicist Madison Grant, who suggested that Nordic people were both superior to, and being overwhelmed by, others.
He not only influenced exclusionary US immigration policies but was an inspiration for Nazis, with Adolf Hitler describing Grant's most notorious book, The Passing of the Great Race, as "my Bible".
Contemporary replacement theory linked to immigration began to emerge in American white power circles and among European anti-immigrant demagogues like British politician Enoch Powell in the 1960s.
The thread was elaborated by post-9/11 Islamophobia propagated by the likes of authors Bat Yeor and Oriana Fallaci, who specifically targeted emergent Muslim communities in Europe.
The contemporary narrative was solidified in the 2012 book Le Grand Replacement by French writer Renaud Camus, who has been cited by several white power terrorists. He claimed white populations around the world were being "replaced" by non-whites via immigration.
In the Donald Trump era, these views have migrated from the fringe into the mainstream, particularly in the US.
The US president has repeatedly trafficked in many of these sentiments, referring to Mexican and Central American immigration as an "invasion" and an "infestation”, loaded terminology intended to dehumanise the people he is referring to.
And the president's biggest media supporters, particularly Fox News channel's evening commentators Tucker Carlson and Laura Ingram, have been force-feeding their audience thinly-veiled great replacement theory rhetoric on a nightly basis for several years.
This explains why the neo-Nazis marching in Charlottesville, Virginia, in 2017, whom Mr. Trump referred to as including "very fine people”, notoriously chanted: “You will not replace us" and "Jews will not replace us”.
Mr Trump and his supporters are enraged that anyone would connect his anti-immigrant rhetoric to incidents such as Charlottesville and El Paso but the direct echoes are undeniable.
Radical Islamism remains a serious threat globally, particularly in the Arab and Islamic worlds.
But in the West, there is every reason to fear that, driven by great replacement paranoia and egged on by the most prominent leaders and major TV news channels, we are entering a gruesome heyday of white nationalist terrorism. The worst is almost certainly yet to come.
Hussein Ibish is a senior resident scholar at the Arab Gulf States Institute in Washington