Think of a post-apocalyptic world: 2% of the population survived, the world is in utter disarray - the works.
The entire populace is controlled by a special group of advanced people, who control all the resources. They live on a special island which houses them and all their special machinery (did I say they were special?). The most important part is a distinct machine which allows them to generate energy. Now, this machine is the sole reason that I have to make them advanced. (I could not have them discover it or handed down from generations).
In this setting, enters an ancient civilization (think elves) who have been put in animated suspension. They are a primitive race of people but they are highly skilled. Their method of warfare is ancient, they fight on horses using bows, swords, spears, etc.
Now in this scenario, I cannot have my advanced race use guns and other modern weapons of mass destruction. So how do I justify the absence of those weapons and when war breaks out between the two, as it eventually will, can I establish a common way of warfare?
Note: After seeing the various answers below, I would like to clarify one point. While it is true that all the violence that has happened up to that current point in the story, would make the populace pacifist but there is a sense of mutual distrust among humans at work too. For example, the advanced people which control energy would distrust the general populace. They would be under the fear that they would be overpowered, their position usurped and their technology stolen. So they have quite strong reasons to support weapons. This is the single strongest reason why I cannot justify it.