Since the emergence in 2013 of the Islamic State of Syria and al-Sham, later Islamic State (IS), followed by its declaration of a caliphate, the British government has estimated that over 850 British citizens and residents travelled to join the group in Iraq and Syria.  Of these, it said that 400 had returned to the United Kingdom (UK), and around 15 percent, or about 120, had been killed. The remainder—around 300 individuals—are still active in Iraq and Syria, or are in neighboring countries such as Turkey. The director general of MI5, the UK’s internal security service, said in October 2017 that to contain this threat, the intelligence services were operating at a scale “greater than ever before,” including running 500 “live operations” and monitoring 3,000 individuals “engaged in extremist activity.”  The authorities have disrupted 13 attacks since June 2013 (BBC, March 6). Nonetheless, since then, jihadists have carried out three significant attacks in the UK, killing 31 people.