In recent history the United States has taken several actions which although they seemed sensible to some at the time have proven to have had serious side effects which denigrate any beneficial effects they may have had. For example, arming the Afghan rebels to defeat the Russians in Afghanistan worked, but it led to the rise of the Taliban and its support for al Qaeda and terrorism against the US. Likewise, selling arms and chemicals to Iraq and Saddam Hussein to defeat Iran (our enemy at the time), led to Iraq's invasion of Kuwait and the current war to get rid of the very "weapons of mass destruction" we helped to create.
Now, with the precedent set by our war against Iraq, any country will be able to justify a war by saying so and so supports terrorism and might attack us, or has murdered its own people, or has failed to obey a UN resolution (which includes lots of countries including the US). The concept of "preventive war" contradicts centuries of diplomatic and legal customs concerning war. There's a reason for following the rules of law although it may seem inconvenient at the time. For example, the recently caught kidnapper of Elizabeth Smart will still have a lawyer and a trial even though everyone "knows" he did it. And even though as in the case of O. J. Simpson, the guilty sometimes do get off, in the long run it's better to have the rule of law because not having it is even worse. It seems clear to everyone except perhaps a certain right wing faction in this country, that we failed to get the needed authority for war with Iraq and I am afraid this will come back to haunt us and the world. For example, why shouldn't Pakistan wage a preventive war against India on the grounds it might be a greater threat in the future?