I wonder if I may move into the 21st century and pose the following questions:
Why have the many acts that have been described as terrorism been carried out. By that I mean the Twin Towers in America and the latest the killings in Paris?
Well, I dislike terrorism and feel there is absolutely zero justification for terrorism, no matter the terrorists' religion, creed, ethnicity, political ambition, race, etc.
However, that said, from what I have read and researched and discovered, terrorists in this specific context as you enumerated say that they are only taking revenge on world governments who carried out mass killings of Muslims in other countries or because of unwavering support of Israel which continues to commit many human rights violations against the people of Palestine with its illegal occupation.
What justification can be offered for such acts?
As a Muslim, I state categorically that no such justification can ever exist for such acts as they are evil acts and Islam has no place for such extremism or adoption of evil acts. Unfortunately, terrorists do not take into account orthodox Islam's position on acts of terrorism and only keep doing the evil they do because what drives them ultimately is a political agenda and utopian ambition of having a Caliphate.
What started this program of violence?
Hmm, I'd once answered this privately and so will paste what I'd written therein here also with minor changes.
The answer:
Global rise in Wahhabism, a particularly virulent strain of extremism, and Salafism to a lesser extent.
Wahabbism, that is, if you are unfamiliar with the term, is an ideological movement that originated in the 18th and 19th centuries in tribal areas of the Arabian Peninsula and was premised on rejection of traditional Islamic scholarship and practices under the guise of "reviving true Islam" and "protecting monotheism." However, what this purification comprised of is extreme censorship in the form of burning books containing traditional prayers, interpretations of law and commentaries on the Qur'an and ahadith (prophetic traditions), and encouraging Wahabbi followers which diverged also into Salafism movement to interpret Islam on one's own regardless of grounding in fundamental Islamic principles and knowledge. Anyone opposed to this new ideology was considered outside of the realm of Islam - an apostate, disbeliever or idolater, thus paving the way for bloodshed and enabling "permissible" confiscation of their wealth. The Wahhabi ideology is especially antagonistic to non-Muslims, and Salafism is too though to a lesser extent. And in the modern era Wahhabism is witness to the adoption of a militant stance against non-Muslims. "Wahhabi" followers and their offshoots in the Salafi movement have taken up an increasingly confrontational standpoint, attempting to impose their ideology in many regions around the world and is funded by Saudi Arabia under the current regime. Under this modern ideological extremism, Islam's essential principle of tolerance and peaceful coexistence with non-Muslims has been abolished and all non-Muslims are believed to be "kuffar" and thereby you and I now are witnessing the attraction some Muslims feel for terrorist organizations like Daesh that are so blithe about their fanaticism.
What books does one need to obtain a balanced view of the events that led to the acts I referred to?
Hmm, to be honest, that's a hard thing to say because the Islamophobic Orientalism phenomena presently seeks to paint Islam and Muslims as the problem responsible for both Twin Towers and Paris Attacks. Of course, Paris Attack is more recent and therefore I'm not even sure what is currently in works in regards to academic scholarship and understanding of this as Daesh is a different organization to Al-Qaeda.
eace:
Least my head aint in no cornflakes 
