Fifteen years after al-Qaida perpetrated the world’s worst terrorist atrocity, targeting New York and Washington, the threat posed by jihadist terrorism remains, casting a constant shadow over international security. This threat changes form from one stage to another, and is set to remain with us for many years to come. The loss of territory by ISIS does not, unfortunately, mark its demise.Jihadist terrorism is first and foremost an idea, and like a virus, the idea spreads, infecting minds, while thriving under certain conditions.A brief history of this virus should begin with Egyptian hard-line Islamist ideologue Sayyid Qutb, who published the book Milestones in 1964, in which he called for the world to be divided into two camps.The first is the House of Islam, where the most fundamentalist interpretation of Islam reigns, and the second is the House of War, which is run by the “unbelievers.”Qutb considered Arab-Muslim states of the 20th century to be fake Western puppets that were pretending to be Muslim, and he said they belonged to the House of War. He was executed by his country in 1966.Qutb’s ideas, however, lived on, and helped foment Islamist uprisings in Arab lands, which were suppressed. In the 1980s, jihadists began flowing into Afghanistan to fight off Soviet occupation.Their ranks included the Palestinian sheikh Abdullah Azzam, the spiritual mentor of Osama bin Laden. Azzam, who was killed in Afghanistan in 1989, called for the establishment of a caliphate to rule the Earth and liberate humanity, while also calling for “disbelief” to be annihilated. Azzam correctly believed that Afghanistan would be the first place where jihadists could enjoy full freedom of operation, and so they did, under the Taliban state that was formed in 1990s. Al-Qaida, using the Taliban as a base, launched the 2001 9/11 attacks, which resulted in the Taliban’s rapid destruction in US-led military action.This marked a new stage for radical Islamists: the stage of homelessness. Osama bin Laden’s al-Qaida was being hunted in Afghanistan by an international coalition, and the jihadists were on the run.Yet the ideas of bin Laden, Azzam, Qutb and others, were spreading fast, this time, on the Internet.These ideas spread all over the world, and jihadist sympathizers began to grow.Some appeared within Muslim minority communities in the West, challenging their parents’ identities, and many more became indoctrinated in the Middle East.The 2003 US invasion of Iraq, and the years of bloody civil war that followed, provided an opportunity for the jihadists to regroup.In 2006, the Islamic State of Iraq (ISI) was formed, and it launched a series of brutal attacks on Iraqi Shi’ites and moderate Sunnis who did not share its goal of creating a jihadist state.Meanwhile, isolated yet devastating mass-casualty attacks rocked Madrid and London. In 2011, the Arab world experienced an earthquake of instability, setting off a domino-like collapse of states. ISI forces began spilling into the imploded Syrian state, filling a power vacuum, and joining the civil war. ISI became the Islamic State in Iraq and Syria – the infamous ISIS. It split off from al-Qaida, and in 2014, it began a new stage, forming a “caliphate.”The vision of a radical dystopian jihadist dictatorship became a reality. ISIS turned into a hybrid entity: a regime with its own army and international terrorist network.It ruled over millions of people and controlled swaths of territory in the Middle East, sending out and inspiring terrorist cells in Europe. It also set up franchises in Libya and Egypt’s Sinai.Now, a new stage appears to be looming.ISIS will soon lose most, if not all of its territory in Iraq and Syria. It looks set to turn into an international network, much like al-Qaida was after the fall of the Taliban, but one that is far more menacing.Those brainwashed by its ideology in Western cities may be more likely to attack at home, setting the stage for an increase in terrorism. In the Middle East, the chaotic and violent conditions that now characterize so many areas mean that ISIS’s ideology will continue to have receptive ears. Its leaders will seek to regain a territorial base, while doing their best to destabilize the Arab regimes they seek to replace.The new and approaching stage does not, unfortunately, represent a victory over the threat.