The Islamic terrorist group dispersed after their strongholds in Syria and Iraq collapsed but rather than being eliminated it is claimed they fannout out into sleeper cells.
Washington-based Institute for the Study of War says they are ready now to fight back and “could be faster and even more devastating”.
In Iraq, ISIS is already gradually seizing control, it states.
The report warns: “ISIS has systematically eliminated village leaders and civilians who cooperated with anti-IS forces.
“ISIS deliberately withdrew and relocated many of its fighters and their families”
Institute for the Study of War
“It has reimposed taxes on local populations in its historical support zones, displacing civilians and de facto controlling small pockets of terrain in Iraq.”
ISIS have used classic guerrilla warfare to fan out their resources, the report warns.
It continues: “ISIS deliberately withdrew and relocated many of its fighters and their families.
“It’s forces are now dispersed across both countries and are waging a capable insurgency.
“ISIS also retained a global finance network that funded its transition back to an insurgency and managed to preserve sufficient weapons and other supplies in tunnel systems and other support zones in order to equip its regenerated insurgent force.”
It claims that ISIS leader Abu Bakr al-Baghdadi is planning a new wave of attacks.
The study said: “The slow motion reduction of ISIS territory and strength initiated by President Obama and continued by President Trump gave the group plenty of time to plan and prepare for the next phase of the war.
“It’s forces are now dispersed across both countries and are waging a capable insurgency.”
There is the potential for even more devastating attacks than those in its 2014 campaign.
It said: “Its external provinces outside Iraq and Syria are contributing resources to its insurgency in those countries while giving the organisation renewed global momentum.”
It added: “Its successful reconstitution of a physical caliphate in Iraq and Syria would produce new waves of attacks in Europe and dangerously legitimise ISIS’s narrative of inevitable long-term victory.”