To continue the analogy, Disney and GM hire the same college grad pool, borrow from the same banks, use the same accounting firms (or law, or advertising, or consulting, etc), deal with related unions. The roots of the companies cross each other. More to the point, the top level execs actually know each other, and exchange pleasantries. That's if they arent doing business together. But they are doing business together. GM is marketing its cars through Disney movies, for example. Superficially there are few points of contact, but under the surface they are part of the same fabric, which we call American Business. If Disney did an Enron and disappeared overnight, GM would feel it.
Saddam Hussien may not have trafficed in terrorism of the type a Qaeda endorses. It has not been demonstrated that he did not, but nothing solid points to it. None the less, they are tied into the same network of arms, explosives and munitions dealers, money handlers and funding sources. Moreover, Iraq was like a Free Parking in Monopoly. When you cannot afford to land on New York or Atlantic, that can be a real life saver. That's gone, and Syria, which was friendly is now less so.
I have spent years watching how news at one business impacts stock prices of others. If you try and trace it, it turns to smoke, but the effect is there. If nothing else, overthrowing Saddam put a bright light in a dark corner, so things can no longer go on unobserved. That is nontrivial in a war of this type. I suspect that there are many more aspects of the overthrow of Iraq that severed resources to al Qaeda and similar groups, but we will never know. Its like a body punch in boxing. you are making it hard for the opponent to breath, even though its almost never a knockout blow.
J
