The U.S. (in the form of the director of the FBI) recently revealed that its efforts to prevent American Moslems from travelling overseas to join Islamic terrorist organizations had worked. That was the good news. The bad news was that groups like ISIL (al Qaeda in Iraq and the Levant) had helped make this happen by urging foreigners who want to join to do so where they are and work to carry out attacks where they live. This advice was particularly useful for ISIL and the foreign recruits in Western nations where young Moslem men not only have problems getting to and operating in a Middle Eastern battleground. These woes (include the difficulty raising the cash for travel and avoiding government scrutiny. When they do get to the combat zone they often have language and culture clash problems. Many of the Western Moslems do not speak Arabic and are unfamiliar with Middle Eastern cultures. That means a lot of these volunteers are only really good for suicide bombing missions and even then they need a lot of coaching and guidance in actually carrying out the mission.
But ISIL (and before them al Qaeda) realized that these foreigners were more useful to the cause if they managed to do jihad (holy war) mayhem in the West. The Islamic terror groups found they could provide plenty of detailed instructions and useful advice on the Internet, often in hidden (dark web) locations that are difficult for the police to find and easy to replace once found if shut down.
This new strategy is causing more headaches and stress for police and intelligence agencies in the West because there are a lot of these wannabe terrorists nearby. A few years you could concentrate on the few who had made some progress in getting overseas or had done so and returned (which is relatively easy to spot). There is no easy way to determine which wannabe will suddenly get it all together and carry out an attack. There have been a growing number of close calls. Also scary was the fact that the November 13th attacks in Paris were mainly carried out by untrained locals using local materials (including well-established weapons smugglers who could provide relatively cheap AK-47s.) Several other recent attacks had the same characteristics and there is real fear that it will all get worse.