Jihad (lit: Struggle/strive) itself is an interesting topic within Islamic history. You are correct in saying that Jihad has always existed. It is explicitly mentioned within the Qur'an as both an inner struggle and an outward fight. It is from here that come the terms Jihad Akbar (Lit: Greater Jihad) and Jihad Asghar (Lit: Lesser Jihad).

Jihad Akbar is, linguistically and historically, the more important aspect of jihad, and refers to the inner struggle to live a piously. This would include things such as avoiding sins and temptations, not being arrogant, and just generally living a holy life. This is the jihad that, in the view of many such as Abdelkader El Djezairi (19th century spiritual/political leader and father of modern Algeria), necessary for all muslims to undertake. It is only if you have tackled this issue of Jihad Akbar that you should move on to Jihad Asghar.

Jihad Asghar is this concept of militarily opposing an enemy, and is seen as a secondary part of the overall jihad struggle. This has always existed, although it wasn't always seen as radical. For example, it was viewed as just and the duty of Muslims to rise up against oppressive rulers. This is what happened in 750 CE with the 'Abbasid rebellion against the Umayyad Caliphate, which was widely viewed as corrupt. This rebellion could be viewed as a form of "jihad". Even after the rebellion was over, there existed roaming groups of khawarij (sing: khariji. lit: those who exit the community) who were determined to stay away from the rest of the Muslims who they viewed as kafr (lit: unbeliever). Some of these extremist Khawarij even attacked Muslim city centers, in the name of killing the apostates.

If you are referring to Islamist jihadist attacks against civilian targets, then it is indeed more of a modern phenomenon. In fact, suicide bombings themselves (especially against non-government/military targets) didn't become common until well into the 20th century. Muhammad himself, although not free of bloodshed in any way, largely opposed the killing of innocent civilians except in certain cases. For example, he slaughtered the men of the Benu Qurayza in Medina after they broke the treaty with the Muslims and helped the Meccans. This was taken as justification by some, such as al-Qaeda, to justify attacks against civilians (you can read about the 7 justifications al-Qaeda used here).

So the justification for and the precedent of the killing of civilians has always existed, at least in a limited fashion. It is the interpretation and acceptance of killing civilians that has changed. Due to the rise of fundamentalist strains of thought from the 19th century onwards, such as the wahhabi movement in Saudi Arabia (I can link to a post I wrote about that before if you would like), literalistic followings of the Qur'an and Sunna (including the belief in offensive jihad) have gained prominence. It is groups that subscribe to these thoughts who carry out the vast majority of terrorist attacks.

I would recommend if you are interested the book The Looming Tower: Al Qaeda and the Road to 9/11 by Lawrence Wright if you are interested in the path of al-Qaeda towards offensive jihad. It is supremely interesting and heavily acclaimed.

Sources

• Bonner, Michael. Jihad in Islamic History:Doctrines and Practice. 2006.

• Kaltner, John & Wiktorowicz, Quintan. Killing in the Name of Islam: Al-Qaeda's Justification for September 11. Middle East Policy Council Volume X, Summer 2003.

• Oxford Bibbliographies. Kharijites

• Lecture notes from university course "Modern Arab Political Thought" and others.