The bombing campaign against ISIL in Iraq and Syria is limited in effectiveness because of the lack of a good informant network in areas ISIL controls. This is largely because most of the territory ISIL controls is populated by Sunni Arabs. ISIL prefers to kill or drive non-Sunnis out of areas they govern. Most Sunni Arabs back the idea of Sunnis, especially Sunni Arabs, being in charge. That belief is so widespread that it’s extremely difficult, and dangerous, for Sunni Arabs to act as an informant against ISIL or any other Sunni Arab leaders.
This situation is particularly acute in Iraq where most Sunni Arabs believe Iraq will not work if Sunni Arabs are not in charge, as they had been for five hundred years. Despite being a minority, since the 16th century the Sunni Turks (until 1918) relied on the Baghdadi Sunni Arabs to help run things in what is now central and southern Iraq. For about a decade after 1918 the British occupied Iraq and also depended on the Sunni Arabs to keep the peace. Then the British left but had to re-occupy Iraq during World War II because the Sunni Arab government (not the king they brought in as part of a constitutional monarchy) tried to ally itself with the Nazis.
At the time many Arabs admired Nazism. The Brits again conquered country, using three divisions and taking three weeks to do it. The Brits found another bunch of Sunni Arab notables and told them they could run things if they stayed away from the Nazis. That lasted for about a decade, until the Sunni Arab politicians and generals decided that this democracy stuff wasn’t working for them. The royal family was massacred and parliament purged of “disloyal” elements.
The Sunni Arabs were now firmly in charge, via a series of dictators, until Saddam Hussein was deposed in 2003. Despite the subsequent elections too many groups in Iraq, not just the Sunni Arabs, believe a dictatorship (with them running it) would be the best solution for the nation's ills. As long as there is the possibility of some group seizing control, Iraq's democracy is in danger. After all, Iraqi had a democracy before (from 1932-58). That one was a constitutional monarchy. There were elections, political parties, and a parliament that passed laws. But it all ended half a century ago when Sunni Arab generals shut down the democracy (actually, they pretended the parliament still worked but the legislators merely followed orders). Saddam ran this military government for three decades and ran the country into the ground. Yet some Iraqis (mostly Sunni Arabs but even a few Shia Arabs) still admire Saddam and consider his blood-soaked reign a "golden age."
Al Qaeda came in after 2003 and added Islamic radical terrorists to all those that the Sunni Arab nationalists had recruited. This backfired, as al Qaeda represented a form of political action that the post–World War II Sunni Arabs had abandoned and even gone to war with. But now, in the name of restoring Sunni rule Islamic terrorists were allowed to do as they pleased. This led to Iraqi Christians becoming "legitimate targets" that should all be killed or driven out of the country. Such threats are nothing new and have been getting worse for over a century. Christians were only 2-3 percent of the Iraqi population in 2003 while a century ago they were over ten percent. The Christians are at most risk from groups like the Islamic State of Iraq (which eventually became ISIL), a coalition of most of the Sunni Islamic terror groups operating in Iraq.
Another nasty side effect of Saddam's overthrow has been the emergence of more major criminal gangs. Some of these existed even in Saddam's police state. Once Saddam was overthrown these gangs largely sided with the Sunni terrorists trying to put Saddam (or some other Sunni dictator) back in charge. The more purely criminal branches of terrorist groups tend to survive, which is how the surviving mafia organizations can trace their lineage back to 19th century freedom fighters. But in the last two decades the mafia and IRA have been reduced to much smaller, and less effective, organizations.
After 2003 the Iraqis used a police approach to terrorism that had worked numerous times elsewhere in the past few decades. India crushed powerful Sikh separatists in the late 80s and early 90s by concentrating on what were basically police methods of developing informers and double agents and going after the key people and the criminal fund raising activities. At the same time, Egypt was crushing Islamic radicals, using similar techniques. Throughout the 1990s, Algeria fought a vicious Islamic terrorist group, finally reducing their numbers from over 10,000, to less than 500. Same thing with Israel's victory over Palestinian terrorists who were successful, for a few years after 2000, with suicide bomber attacks inside Israel. The U.S. adopted a lot of the Israeli techniques for intelligence collection and agent development.
Iraq had another advantage in that American and Iraqi counter-terrorism efforts had managed to tear up the Islamic terrorist groups by 2008. Many Sunni Arab terrorists accepted (with some trepidation) various amnesty deals. Al Qaeda, which was still largely a foreign outfit, has been crippled with the killing or capture of most of their senior leaders. Being foreigners, and favoring attacks on civilians, made al Qaeda the most hated group in the country. There were plenty of tips from concerned citizens because of that.
Iraqi members of al Qaeda then switched to criminal gangs, relegating Islamic terrorism to the "what I do in my spare time" category. While the U.S. contributed lots of essentials (UAVs, intelligence collection, and analysis) support for the counter-terror battle, the Iraqis did most of the work on the ground. The Iraqi cops took advantage of the fact that most Iraqis wanted peace. Three decades of Saddam's misrule and nine years of post-Saddam terror created a widespread desire for less unrest. While there are far fewer terror attacks after 2008 (less than ten percent of those during 2004-7), they persisted, and police believed there were enough diehard Islamic radicals and violent criminals to keep the bombs exploding well into the next decade.
After the Americans left in 2011 (because the Shia politicians could not agree on a Status of Forces agreement to protect Americans from the corrupt Iraqi police and courts) they took their intelligence specialists and equipment with them. The Sunni Arab informants who had worked for the Americans generally refused to work for the Shia controlled Iraqi government. The Iraqi Sunnis did not trust the Shia government, in part because many Sunni were quite open about expressing their belief that eventually the Sunni minority would be back in charge. The expensive and hard fought peace fell apart and now it’s back to the bad old days of the Sunni minority seeking to dominate the Shia and Kurd majority.