I think the original justification for invading Afghanistan was that it was harbouring existing and potential terrorists. According to that that thinking we should, presumably, invade Pakistan, Saudi, Yemen, etc. etc.
There is undoubtedly widespread resentment throughout the Arab/Muslim world about the Israel/Palestine situation, but there also exists a certain number of fundamentalists who wish to impose Islam on us all. They won't be stopped by military means, only through education.
The USA wanted revenge for 9/11 and Britain wanted to please the USA. When will they learn that you cannot succeed by force in a country where the bulk of the population are not on your side.