Generally speaking (no pun intended

) - the Department of State and the Department of Defense are at opposite ends of the methodology in forging and implementing foreign policy.
Sometimes the military is used in conjunction with statecraft, when it is purposefully used as a threat. But, the actual use of the military represents to the State Department people a recognition that statecraft has failed. War results when diplomacy fails. When the Department of Defense takes over it's because the Department of State did not accomplish the task, again generally speaking. This is not to fault the State Department - it may not have been their fault. As Secretary of State I bet Colin Powell would much rather have been successful.
Secretary Powell is in a unique position of not only having served on the battlefield himself as a soldier, but also as a commander of people. In the upper echelons of the military, command level, there is as much diplomacy at play as "I order you to do this." Often a general at this level is as much a diplomat, as much a "people person" as a commander of people. A true commander doesn't just order people - he leads them. To do that he must have earned their respect and trust.
It was not by accident that General Eisenhower was where he was. He was a consummate politician. Who else could have gotten along with Churchill, Montgomery, Roosevelt, Marshall and De Gaulle?
Secretary Powell brings a unique perspective and set of talents to his position.
Whether you have seen the proofs you require to change your position - is of course your personal right. But apparently he has seen proofs to change his position. That's of interest to me.