The word terror connects with the anxiety that comes from violence that targets civilians. A bus or restaurant that a suicide bomber selects for his or her sacrifice, a roadside bomb laid by someone who chooses to kill without dying, or a group of school kids or supermarket patrons held hostage in the name of some cause, 
There is no end of examples that we've experienced first hand or via the media in recent years.
We've been lucky never to have been closer than the sound and then the smell of an explosion, sirens that began within a couple of minutes, and blocked streets.
It's not a new phenomenon. European anarchists used terror in the 19th century for the sake of class or ethnic causes. Further back, we can find it as a tactic of Jews against the Romans and against Jews who collaborated with the Romans in the first century.The French Revolution had a reign of terror. It was employed in the US against Blacks by White supremacists from the 19th century to last week, and by Black separatists against their enemies.
The concept is inherently political. One person's terrorist is another's freedom fighter.
Islamic terror is especially problematic. The concept generates anxiety among those who view religion as outside of political dispute, do not want to upset decent Muslims, or provoke a wider uprising among a billion people who adhere at least nominally to the faith.
Terror's use by non-governmental gangs raises problems not apparent in wars between nation-states.
They are almost enough to make us yearn for the simplicity of a great war between entitles, each with their own government.  Estimates of casualties for World War II range to 60 million dead, but the principal adversaries were nation states whose governments could end it with a surrender. 
Islamic terror has the support of emotions and recruitment potential associated with religious doctrine. Islamic doctrines include those as humanitarian as anything in Judaism or Christianity, but also those that glorify violence, self-sacrifice, and eternal reward for fighters in a Paradise that some believe is populated with lots of virgins willing to serve the cause. There is also violence in the doctrines of Judaism and Christianity, but they have been quiescent among Jews since ancient times, and among Christians since the Holocaust.
Islamic terror has the support of governments, but depends on non-governmental movements, organizations, or gangs undisciplined by anything approaching the institutions of a state or government. Rivalry, animosity and warfare between the clusters is more apparent than unity, cooperation, or anything that looks like a hierarchy peaked in a person or institution with the authority to make policy. Casualties, destruction, or apparent defeat of one cluster is not likely to produce the surrender of others. 
What's happening in Syria illustrates better than anything the problems in combating Islamic terror.
By one count, which may have become obsolete shortly after it was published, there are 30 organizations of fighters. Competition and warfare between the groups is more apparent than cooperation. Each has its sources of funds and munitions, campaigns for the recruitment of fighters and support personnel, along with reasons for existence and a leading figure or cadre. Motives for their rivalries combine themes of theocracy as well as politics and the egos of charismatic figures. 
Israel's struggles with Gaza illustrate the problems of dealing with non-governmental organizations that claim to speak for a people, but use civilians as cannon fodder. IDF attacks against neighborhoods and buildings from which Israeli civilians and troops are attacked are called to account for targeting civilians, or not showing enough regard for civilian casualties. 
Unlike governments whose armies have been crushed, terrorist organizations do not cry "Uncle."
Hamas rules imperfectly in Gaza. It claims victory in the violence of 2014 despite 2,200 deaths and thousands of structures destroyed, much destruction left unrepaired from the previous spurt of conflict, and little of the money promised for reconstruction this time actually delivered. 
Hamas now claims to be refraining from missile attacks against Israel, but asserts that it lacks control over more radical competitors.
Among the consequences of World War II is a host of inter-governmental and non-governmental organizations claiming to stand for human rights whose spokespeople demand standards of military conduct unattainable in anything like Israel's conflicts or those directed against Islamists by the US and its European allies, Egypt or Saudi Arabia.
Barack Obama could declare a day of celebration with the death of Osama bin Laden, but there were others to take his place, and his killing provided stimulus for those seeing the United States as the source of evil.
Also involved are historic fissures in Islam, with the Sunni-Shiite being the most prominent but not the only reason for warfare against enemies both outside and within the Islamic tent.
Who can surrender and impose such a decision on those who want to keep fighting?
In the absence of an answer, commentators and generals involved in the conflict see a war extending for generations. 
Optimists see hope in the fractures within Islam, and an Egyptian-Saudi front useful against Shiites inspired and supported by Iran, or against Sunnis committed to battle any governmental establishment.
None of which helps with the agenda of human rights organizations, and politicians inspired by them, who seek to democratize the Middle East and develop judiciaries operating according to the standards of London, Paris, and Washington.
Those with such aspirations should look at Iraq 12 years after George W. Bush's insertions of democracy among his reasons for invasion, and the real Middle East 6 years after Barack Obama's Cairo speech that supporters saw as leading to a New Middle East.
Then there are the problems of South Carolina, as well as all those other gun tooting characters with motives that may be neither religious nor racial..
Advocates of gun control in the US must come up with a scheme to deal with who knows how many guns in private hands. Estimates range into the hundreds of millions.  
Lots to do before it's a perfect world.
No easy tasks.
Given the nature of the problems, I prefer to limit my criticism of those who try.