Dumbledore has no difficulty with action when needed(TSPE, most notably), but he's been burned too many times by the cost of his efforts to be eager about it. He'd much prefer to stop the war by passive deeds(sequestering Harry, poisoning Voldemort's father's grave, etc.), and not risk the bloodshed that open war would cause, or even the loss of political capital caused by a showdown with Lucius Malfoy. There's bound to be a big difference between an 11 year old sci-fi fan and a hundredish year old veteran when it comes to eagerness to do harm, and frankly I think that Dumbledore's caution is at least as justified as Harry's sneakiness when it comes to planning a war. After all, Harry's never seen one of his incredibly clever plots fail, and he's eleven, so he is naturally going to be far too eager. Inaction isn't always wrong.