Close your eyes and imagine this: two nations on the brink of battle. Each side evenly matched and ready to spill blood. Their shared border looks like a giant military camp, with both side warily watching the other. Suddenly, a stray arrow hits a commander or a command is misinterpreted and the battle is joined! For years, blood fills the land and black clouds signal the death of many villages on both side. In the course of time a neutral great power not adjacent to either side negotiates a fragile peace. The smallest incident could shatter the armistice.
The great power, stretched thin in other wars, recruits the PCs for a small task. Keep two highly motivated, extremely well armed nations from killing each other, without spilling any blood. Of course, both sides hate the PCs and would love dearly to start killing each other again. Even if it is over the PCs cooling corpses.
Recent events, such as the attempted assasination/coup of the East Timorese president and the official secession of Kosovo from greater Serbia inspired me. Both are regions of intense historical bad blood and potential for great violence. Violence that can appear in an instant. A single mistake by a single lowly foot soldier can literally start a war. Fighting a war is easy, preventing two nations who hate each other from starting one is very, very hard.
Have fun dropping your PCs into this nightmare. To any players reading this, please do not blame me. I just came up with the idea, your DM implemented it.
Trask, the Last Tyromancer