Use of force between organizations falls into two categories, deterrence and compellance. Deterrence is the use of the threat of pressure to influence another party's actions. In other words, it makes the costs of a course of action outweigh the gains. Compellance is the application of pressure on an opponent to influence their actions (rather than the threat of pressure).
In this sense, warfare is a very specific subset of communication in which deadly force is used in either a deterrent or compellant role to influence behaviors of an opponent.
Good so far? Ok.
In any strategic contest, the contest may be either symmetric or asymmetric. In a symmetric contest, both parties are choosing to prosecute the contest of wills on the similar terms and in the same arena (e.g. Syrian and Israeli jets slugging it out over the Bekaa valley in '82). In an asymmetric contest, opponents choose to engage each other on dissimilar terms in the hopes of obtaining a decisive advantage in the contest of wills. The current Israeli-Palestinian conflict is an example of this - Israeli AH-64 attack helicopters performing strikes against Hamas leadership targets versus suicide bombers blowing up cafes. Another example of a warfighting asymmetry was the use of longbows at Agincourt to defeat French knights. Or the use of bulls to repell the Spanish invasion of the Azores. Or the use of nukes to creat a shock and awe effect in the Imperial Japanese government. Historically, asymmetric conflict has been far, far, far more common than symmetric conflict.
Ok, I just cut a whole lot of history out here. But even I get tired of my bloviations.
By the dawn of the twentieth century, two things had come to light - soldiers fought soldiers on the battlefield and soldiers attacked means of production behind the front lines (sort of like the distinction between tactical and strategic bombing). It is important to note, however, that even then creation of civilian casualties (collateral damage) was never considered an objective, in and of itself. People were attacked directly with the intent to kill individuals when they were soldiers, while behind the lines, attacks were centered on production.
As conventional warfare has become much more lethal in the last several decades, sensitivity to collateral damage has become much higher (in the conventional warfare arena). First, with the increasing lethality of modern warfighting, the entry cost for those wishing to fight a conventional symmetric war has gone through the roof. No longer can you convert a tractor factory to a tank factory and produce tanks with a prayer in hell of standing against top-line armored formations. As a result, the gulf between the combatant and non-combatant has increased in modern armies. Combined with the ability of modern weapons to start racking up body counts, civilian casualties have become a very, very big no-no.
So people have resorted to pursuing more asymmetric responses, such as low-intensity, low-tech conflict (like VietNam), low-intensity, high-tech conflict (like the no-fly zones and "cyber terrorism"), and high-intensity, low-tech war (like Congo, Sierra Leone, Liberia, et al.) These asymmetric responses have in some cases dropped some of the traditional conventions of warfighting.
Part of this has been the advent of terrorism. Now we all are more or less familiar with Guerilla Warfare, in which either troops or irregulars conduct high-mobility attacks on military forces. That's considered fair game and part of the cannon of modern warfare.
The new twist is terrorism. Terrorism has been around for a while (e.g. Jewish terrorist attacks during the British occupation of Palestine). State-sponsored terrorism, however, didn't really come into its own until after 1967. It became a mechanism for states who recognized their marked conventional warfighting inferiority to continue to prosecute war through proxies against various targets. (Guerrilla warfare carried with it a much higher risk of evolving into conventional war, which is just the thing some states really wanted to avoid).
What makes terrorism such a hot-button issue is that it violates two of the "customs" of modern warfare. First, the practicioners do not distinguish themselves as combatants. That is frowned upon, but happens, as in VietNam or Somalia. Second, they target civilians for the simple sake of killing civilians. That too is frowned upon, as in Rwanda, Kosovo, East Timor, et al.
Terrorist attacks are a form of compellance that directly, with forethought and intent, does two of the "don't do" things. Full stop.
Why do we get so darned exercised about it?
Because in this particular asymmetry, we "play by the rules" we try to make sure our soldiers marked clearly as combatants and we try to avoid collateral damage. The "bad guys" try to make their combatants blend in and have no, and I mean absolutely none whatsover, concept of collateral damage as being distinct from intended damage.
One can see how this clearly gets some folks rather exercised.
But here's the kicker, while we continue to prosecute our strategic conflicts according to "the rules" - the other guys don't, but are held as being equally moral in the prosecution of their grievance.
The reason this is bad is the kind of human behavior we saw in the Pacific in World War II. Originally, American soldiers both felt that they were able to surrender and accept surrender if the situation demanded it. Just like in Europe or North Africa.
After the, ahem, "cultural misunderstandings," that attended surrender in the Pacific, the entire theater essentially went to a "Take No Prisoners" status for both sides. In other words, an asymmetry or prisoner treatment very quickly developed into a symmetrical treatment.
Taking that historical analogy, those supporters of terrorists may very well encounter the day that the U.S. cares as much for civilian Arab casualties as the Palestinian street cares for the dead of 9/11. That, clearly, will result in a losing strategy for the Middle East and Islam as a whole. While currently the entire Arab world has a GNP lower than that of Spain, the U.S. spends roughly 45% of the world's defense budget. If the terrorists' insensitivity to civilian targets doesn't change (and they consider paying U.S. income tax a sufficient reason to kill you) then this does stand some chance of metastasizing into a something that will look a hell of a lot more like genocide than the many on the Arab street can possibly even begin to fathom.
And not because westerners are racists or this that and the other, but simply that every time one opponent adopts an asymmetrical approach, they damned well better understand that their opponent may choose to break that asymmetry and may just do it better than you. And don't scoff, that very same mechanism is exactly what kept the Cold War cold, chemical weapons out of World War II.