The events of September 11th, 2001, can be called the 'impetus' for the pair of wars we have today only by closing our eyes to the realities of the last hundred, even last few thousand years. To try and untangle the muddle would almost certainly leave out enough events to be insulting to one side or the other. The attacks in New York were claimed by their perpetrators as a response to American imperialism and interference in the Middle East, in the form of support for the Israeli state against the Palestinians. American imperialism and interference, in turn, is a response to the Middle East's stranglehold on the world's oil reserves and its willingness to cover for the crimes of Islamic extremists in Israel and elsewhere. The back and forth could be traced all the way to the sacking of Jerusalem, or even the prophet Muhammad's conquering trail through the Arabian penninsula.
The point is that at some point, you have to take an event on its face. A political actor must accept that he is the perpetrator of his own actions, especially when what he undertakes is in the name of those who are long dead and far beyond having their actions 'redeemed.' The attacks on September 11th constitute such a point. It was a raise of the ante, a deliberate act of cruelty intended to change the nature of the game, and it did. It made clear that there were forces that were willing to reach around the globe, to carve 'You are not safe' upon the great American Experiment, and to involve deliberately an American population that, for the most part, long since relinquished control over its political and military machine. Whatever can be said of how much it awakened the population, it has changed the way politicians interact, the way states interact, and the way people interact.
Americans have a love of liberty. They're raised with it from cradle to grave, and only a few - immigrants, brave journalists, long-deployed soldiers, and refugees - know what it is not to have guaranteed basic rights like the ability to speak your mind in public, to demand redress of grievance from the government, to defend yourself against unwarranted government intrusion. There is a general feeling, poo-poohed by liberal intellectuals as 'cultural relativism', that American freedoms should be the way the entire world works. There is a feeling that the guarantees in the American Bill of Rights and the United States Constitution are somehow correct, that they form some cornerstone of personal and political authenticity. To export these freedoms to others seems worth the price to many people, so long as the moral intentions are clear and open.
Humans all over the world possess a similar character; no human being, not even one of the leadership caste, can truly prosper in a totalitarian famine-state. Similarly, a life spent in lawless jungle villages, constantly fleeing in terror from roving squads of murderous thugs, losing precious material objects or even family members along the way, has little to recommend it. This is especially true when compared to the very real chance of a life of material wealth, intellectual and cultural indulgence, and personal freedom of movement, assembly, and expression. When we see other human beings in such situations, we are moved. An individual have almost no power to affect even a democratic country's decisions, aside from a single vote in elections that are increasingly meaningless. But when the country mobilizes for a war to root out a man whose stated goal is to rid the world of 'America' (and breathe deep of that particular statement: Here we have a man born of Saudi royalty, whose organization and its sibling organizations have consciously increased the violence in their home territories, whose religious beliefs include the intellectual and social stunting of half the world's population, and whose law stems from a barbaric and Iron-age belief in mob rule and retaliation, calling for the destruction of the only state to guarantee freedom from precisely such things in its foundational documentation) and expands it to include ousting a tyrant whose means of subjugation has included roving political death-squads, deliberate starvation, and crimes against humanity limited only by the lack of larger-scale technology, what's not to like? What human being, especially a morally interested American, would not be in favor of liberation?