areas in Syria and Iraq attracted particular attention and voluminous analysis, though they represent a smaller percentage of the total number of FTFs.16 As regards the return or “reverse flow” of FTFs, which has taken place in waves, reliable statistics are, again, elusive. One report tracked 5,600 fighters who had returned to their home countries globally by 2015.17 Another report, focused on the FTF issue in the European Union (EU), suggested that some 30 per cent of FTFs had returned or moved to other states by 2016.18 The “shrinking territories” in Syria and collapse of the so-called “Islamic State” caliphate in Iraq in October 2017 contributed to the latest wave of returnees.19 By early 2018, evidence pointed to both a sharp decrease in the number of those travelling to Iraq and Syria and to growing concern regarding the impact of FTFs returning to their home states or moving on to other ��� sometimes unknown – locations.20 In light of these dynamics, concerns that ISIL is turning its sights elsewhere, potentially using returning FTFs, has been a defining feature of the political discourse and related developments in law and policy. The widely reported involvement of several former FTFs in attacks in Brussels, Paris, Istanbul and London between 2015 and 2017 appeared to confirm this fear. Concern has also arisen from research suggesting that FTF-related attacks, while infrequent, have been particularly brutal and lethal.21 At the same time, others have called for some perspective on the relatively very small number of FTFs who have engaged in any acts 16  “Terror Situation and Trend’ report (TE-SAT 2016)”, Europol, 2016, p. 26, <www.europol.europa.eu/sites/ default/files/documents/europol_tesat_2016.pdf>. Europol writes that “More than 5,000 Europeans are believed to have travelled to conflict areas in Syria and Iraq”. The International Centre for Counter-Terrorism (ICCT) cited the number of between 3,922 and 4,294, hence on average 4,108. See: B. van Ginkel and E. Entenmann (eds.), “The Foreign Fighters Phenomenon in the European Union: Profiles, Threats & Policies”, International Centre for Counter-Terrorism, April 2016, p. 4, <www.icct.nl/wp-content/uploads/2016/03/ICCT-Report_ForeignFighters-Phenomenon-in-the-EU_1-April-2016_including-AnnexesLinks.pdf> (hereafter, B. van Ginkel and E. Entenmann (eds.), “The Foreign Fighters Phenomenon in the European Union”). Tunisia is believed to be the largest source state, with 6,000 persons estimated to have travelled to the conflict in Syria from that state alone. See: “Foreign Fighters: An Updated Assessment of the Flow of Foreign Fighters into Syria and Iraq”, the Soufan Group, December 2015, p. 15, <soufangroup.com/wp-content/uploads/2015/12/TSG_ForeignFightersUpdate3.pdf>. 17  “Foreign Fighters: An Updated Assessment of the Flow of Foreign Fighters into Syria and Iraq”, the Soufan Group , op. cit., note 16. Although still cited in 2017, news reports and other sources (e.g., UNSC meeting records of 28 November 2017, SC/13097), the number can now be expected to be significantly higher. 18  B. van Ginkel and E. Entenmann (eds.), “The Foreign Fighters Phenomenon in the European Union”, op. cit., note 16. 19  “The Challenge of Returning and Relocating Foreign Terrorist Fighters: Research Perspectives”, UN CounterTerrorism Committee Executive Directorate (UNCTED), March 2018 (hereafter, UNCTED Trends Report 2018), <www.un.org/sc/ctc/wp-content/uploads/2018/04/CTED-Trends-Report-March-2018.pdf>. The current wave of returning FTFs is described as being larger and more diverse than previous ones. 20  Many FTFs remain unaccounted for, having either died overseas or moved to unknown locations. See, for example, UNCTED Trends Report 2018, op. cit., note 19. See also: “Risk Analysis 2017”, Frontex, 20 February 2018, pp. 30-31, <frontex.europa.eu/assets/Publications/Risk_Analysis/Annual_Risk_Analysis_2017.pdf>, (hereafter, “Risk Analysis 2017”, Frontex); “Now that the Islamic State has fallen in Iraq and Syria, where are all its fighters going”, the Washington Post, 22 February 2018, <www.washingtonpost.com/graphics/2018/world/isis-returning-fighters>; D.L. Byman, “Frustrated Foreign Fighters”, the Brookings Institution, 13 July 2017, <www. brookings.edu/blog/order-from-chaos/2017/07/13/frustrated-foreign-fighters/>. 21  See: UNCTED Trends Report 2018, op. cit., note 19; Hegghammer, “Should I stay or Should I Go? Explaining Variation in Western Jihadists’ Choice between Domestic and Foreign Fighting”, American Political Science Review, Vol. 107, No. 1, (February 2013), p. 11. 12 Guidelines for Addressing the Threats and Challenges of “Foreign Terrorist Fighters”