I struggled a long time with how to say this. It's really the point of the series and serves as both a reprimand of Republican Hegemony and a warning to our own side about how we need to move forward in the future both at home and abroad.
The problem I identify with power, both for the actor and acted upon, is a lack of control. In Part I, I talked about the consequences of a powerful actor using its power unilaterally. It breeds a feeling of lack of control for the acted upon that can lead to very bad consequences, and uses up a lot of resources for the actor.
In Part II, I talked about the difficulties for the actor in terms of the perceptions of power as opposed to more empirical consequences.
In Part III I will talk about a more abstract danger directly to the actor in regards to using power. The lack of control. This leads to the corruption.
When a Hyper Power uses power, they create consequences and so on and they have to react to these consequences. This results in having to use power to deal with those consequences. You can see how this works by looking at Iraq. You use your power to invade, then you have to use your power to try and rebuild the country (or take its resources) and this results in having to expend more power in pacifying it so you can rebuild, in trying to solicit funds or manpower from other countries and so on. You have to spend huge amounts of both political capital and resources to manage the consequences of your invasion--and contrary to what might seem obvious, you are not in control. You have set things in motion that you can no longer ignore and so, you are trapped.
That's what happens when you use power. It traps the user into certain paths, closing off doors of opportunity until further actions are no longer a question of "should" something be done but "can" it be done. CAN we pacify Iraq? CAN we increase troops levels? CAN we create a functioning political process? CAN we prop up Maliki? We can no longer spend time on the "should" because we no longer have the option of very many choices.
The only way to nullify this is to try and nullify the initial act, in this case the invasion of Iraq--by pulling out. The only way to regain any kind of freedom is to STOP USING OUR POWER, but that means that you have to accept diminished power as a consequence of regaining your freedom.
Look at another example, Karl Rove's 50%+1 strategy. He chose to exercise the power he helped wield to create that kind of country, subordinating everything into that goal and it created the batshit insane constituency that still approves of Bush and trapped him into having to keep on demonizing opposition and pursuing things like Schiavo all to deal with the consequences of the strategy. Now look at him, Bush is completely trapped.
These problems don't happen (at least they do not happen as badly) when you don't have the Hyper Power. When you are not a Hyper Power and you exercise power, there is someone around who is more powerful and that means they can stop you. In many ways you are free to act because someone else will deal with the consequences. As a Hyper Power there is no one else you can shove this off to, if you screw it up when using power you and you alone have to take it in the teeth if you act alone.
When acting multilaterally, what you have at most is a "first among equals" situation if only superficially. As illustrated when I talked about financial consequences, this spreads the pain around so when you stop using your power, the consequences to any particular individual power are blunted. Another aspect of multilateral action is because you are equal and can at least in theory all stop each other your exercise of power can be checked so you can maintain more freedom of action.
That's why power corrupts people, because use of power leads into the power making the decisions for you by transforming conversations into "can" instead of "should." It creates a lack of control that forces the user to lose any kind of ethical/moral calculus in their actions without suffering extreme hardship.
The lesson to take from all this is that power should be used sparingly and only after a lot of deliberation. Sometimes you have to act, and sometimes you have to act alone, but because of the consequences both to yourself and others, acting multilaterally might not be as expedient, but it ultimately serves everyone's interests in the long term.