Leaving aside the old saying that a terrorist is “a patriot without an army, navy or air force”, the 'rational' - but never justifiable - purposes of terror include, but are not limited to:
forcing the electorate of a state that is oppressing other less powerful states and their people to confront the fact that they cannot escape the consequences of the regime they voted for, or allow to remain in power:
terrorising the civilian population:
gaining publicity for their cause:
forcing the government of the state being attacked to resort to more and more repressive measures and the progressive removal of their own people's democratic rights and freedoms.
The irrational 'purpose' of terror is to give violent expression to feelings of impotence and frustration at perceived injustices perpetrated by a more powerful enemy.
Every state, every empire that has ever existed has used terror at one point or another in its history, and the bloody policy continues. If it is large-scale, it is called military intervention, pre-emptive strike or war – if it is small-scale, it is called covert intelligence operations.
If I revisit the objectives of terror I outlined above, I have to say that Al Quaeda has been successful beyond its wildest dreams - and they are wild and inhuman dreams - because they have produced exactly the effects desired by the terrorists. They have created a bogeyman, and we - the West - have swallowed the fantasy whole and entered into their paranoid nightmare, one from which the world may not emerge.
Unless sanity prevails, we are in a state of perpetual war against a shadowy enemy that cannot be defeated by violence. Only a systematic removal of the manifest injustices that exist in our world by patient, sustained action can eliminate terror, and a good place to start is the Israel/Palestine conflict that is eating like a cancer at our morality and our identity.
I am against violence as an instrument of political policy, however it is applied and on whatever scale. But I am not a pacifist - I believe that an individual or a state has the right to use violence to defend itself against an aggressor, and I believe in the concept of a just war.
Perhaps WW2 was the closest the world ever came to a just war - it was certainly America and Britain's finest hour, and Britain has a debt of gratitude to America than can never be adequately repaid.
But it cannot be repaid, even in part, by failing to speak out against the appalling injustices of both American and British foreign policy as they have existed for the last half century or more. The concept of defence, the concept of a just war, and any relics of the morality and statesmanship, not to say common humanity that existed between 1939 and 1945 have been debased almost beyond recognition.
America now has a man who I believe will prove to be, not only a great President, but a great world statesman, of a kind rarely seen more than once or twice in a century. Like many millions of Americans and non-Americans, I have placed my hope and trust in Barack Obama, because there is no politician in Britain of remotely comparable moral and intellectual stature. But his enemies are many, and highly vocal and powerful. In these dog days of the British Empire, I also place my faith in a small country – Scotland - that has no intrinsic power except the power of its intellect and its citizens' belief in a better world – and a non-nuclear world.