Those who believe Bitcoin is the solution to all the worlds woes are setting themselves up for disappointment. War existed before fiat currencies, and war will exist without fiat currencies. While we like to think of ourselves as civilized creatures, we often find ourselves unable or unwilling to solve our problems without having to resort to war.
What Bitcoin can do, though, is reduce a governments ability to go to war. If a government is unable to print off new money in order to fund a war effort, they would be forced to rely on their reserves. This would make the decision to go to war a far more difficult one, not only for governments but also for the common people.