[Pitchwife]

Quite the contrary, I'd say, Marky. If he was running off from one important battle (which he knew would be taken care of by Gandalf and Théoden), it was only in order to run towards another, no less crucial battle. He knew about the Corsair fleet approaching Pelargir via the Palantír, and that had to be dealt with as well. The Paths of the Dead were his only hope to not only reach Pelargir in time but also raise an army on the way that was strong enough to counter that threat. True, it was a dangerous path, but as Esty has pointed out, he knew he was meant to take it and he, as Isildur's heir, was the only one who could command the Dead. Anything but a military mistake!