The killings by Muslims clearly have been inspired by anger at the killing of Muslims in the Middle East. The terrible death toll in the US-led wars in 7 Middle East nations over the past 15 years was surely a major factor. Only very rough estimates are available. In Iraq alone, somewhere between 100,000 and 500,000 (possibly more) people were killed in the period 2003-2011 . In Afghanistan, since 2001, over 26,000 civilians have been killed as a result of US led actions . Hundreds of Pakistani civilians, including scores of children have been killed by American drones, many in attacks on wedding parties . Libya and Syria have been devastated by American bombers and missiles, with thousands of people killed, wounded, or rendered homeless.
This is not to suggest that the terrorist attacks by Muslims were justified in any sense. Killing or injuring innocent people is always a terrible crime; explaining why such crimes are committed in no way condones them.
The 9/11 attack by an extremist group was said to be a response to the stationing of 5,000 American troops in Saudi Arabia, and to the US role in arming Israel. The US has a long history of terrorist behavior, starting with the genocidal attack on American Indians, and continuing with such brutal adventures as the conquest of the Philippines at the start of the twentieth century, and various killings in Central America .
When the population of a nation is assaulted, it is likely that some people there will seek vengeance. France joined the US in bombing Syria in 2014. In retaliation, a Muslim group attacked Paris in 2015 . Immediately, the French responded by bombing Islamic State targets in Syria, which included a hospital, clinics, a museum and other buildings in an urban area .
What happened in France exemplifies the penalty for thoughtless brutality. After that retaliation, it would have been more appropriate for the French people to direct their anger to those in their government who, by joining the attack on Syria, provoked the avenging attack. Instead, by joining in additional US-led attacks on Syria, the French have made themselves potential targets for further retaliation.
As an aside, note how the killing of an identifiable person by a Muslim is almost always characterized by our news media as terrorism. But such language is never used to describe such acts as the killing or mutilation of a dozen Muslims, including women and children, by a bomb dropped from an American airplane. A drone pilot, located in Nevada or New Mexico, who kills people, including children, in Pakistan, is not considered by the news media to be a terrorist. Nor are those who ordered such attacks.
Perhaps the real surprise is that there has not been an organized attack by Muslims on US soil since 9/11/2001. Some might argue that this is because the FBI, CIA, NSA, and other such agencies have done a great job in defending us. But there have been no credible accounts of the thwarting of significant plots. Rather, there have been many instances in which government agents entrapped young Muslims into incriminating themselves in crude terrorists plots. There have been studies showing that airport security is very porous . This despite the fact that the annual federal budget for airport security is about equal to the total budget of the FDA (Food and Drug Administration)--about $5 billion.
It appears that, thus far, no Islamic terrorist organizations in the US are organized well enough to plan and execute an operation anywhere near as complex as 9/11. But there is no guarantee that this situation will not change. It is not feasible to stop terrorists from entering the country, or to ensure that they would be detected after arrival. It is easy to imagine an attack, perhaps in a large sports arena, by 10 or 20 people, resulting in over a thousand deaths. The likelihood of such an event grows with the growing anger about the large number of victims of US attacks on people in the Middle East and elsewhere.The fire next time
What would be the likely aftermath of such an attack in the US? One possibility is that Americans would suddenly realize how painful it is to be on the receiving end of a bombing, and would vociferously demand an end to violent US actions all over the world. Sadly, that is very unlikely to be a common reaction. Instead, our government would almost certainly launch one or more "retaliatory" responses against nations that could somehow be blamed for the attack. These attacks would serve as recruiting mechanisms for further terrorist attacks on the US, etc. Many Muslims in the US would be rounded up and interned, and others assaulted by mobs. Additional laws and regulations enhancing the power of the police, FBI, CIA, and other government agencies would surely be enacted. We could anticipate major assaults on the Bill of Rights, expanding the damage already done.Who started this terrible cycle of violence?
Each side claims that their attacks are either in retaliation for attacks on their nation, or attempts to prevent further assaults. But the situation is not symmetric. The US had never been bombed or invaded by government forces of any Muslim nation. Of the 19 terrorists who carried out the 9/11 attack, 15 were Saudis, 2 from the Union of Arab Emirates, one Egyptian, one Lebanese. It is interesting that none were from Afghanistan, Iraq, Libya, Syria, or any of the other nations that were targets of the US reaction. Americans have wreaked havoc on a number of Middle Eastern nations. Primary responsibility for the current situation is on the shoulders of the US, and so it is up to the US to take the initiative in ending the cycle of violence.References
 Wikipedia, "Terrorism in the United States", Wikipedia
 Peter Bergen, "Can We Stop Homegrown Terrorists? Wall Street Journal", Wall Street Journal, Jan 22, 2016
 Zach Dorfman, "What drives Americans to join the jihad? The book 'United States of Jihad' explains", Los Angeles Times, Jan. 28, 2016
 Dan n Vergano, "Half-Million Iraqis Died in the War, New Study Says Household survey records deaths from all war-related causes, 2003 to 2011", National Geographic, Oct 16, 2013