If you watched any seasons worth of Game of Thrones you probably heard Littlefinger give this line. It boils down to a pretty simple meaning: when everything is in chaos, you can climb. In peace, the line of succession for King is a set and known thing. In War who is King, who has Noble blood, and who deserves to lead can change a great deal just from the results of a few sword swings.
The interesting thing about this saying in fiction and RPGs is that it tends to be true for two categories of people: the heroes and the villains. In RPG terms this tends to boil down to the Big Bad Evil Guy/Gal, and the PCs. Of course, if you point this out to most players they balk. They earned their way up after all - and they did. But the opportunity for them to do so came out of chaos, and chaos is something that oft follows in the wake of PCs.
Men Who Want More
Women and Men who want more, and are willing to set things into chaos to get more, often end up on the side of villains. They interfere with the way things are done, sew dissent, cause war. They play one side against the other as best they can until things are in enough chaos that they can seize the next rung on the ladder and climb from there.
The problem then is that once they have power, you still have chaos. You have to calm things down. Then of course there are those who will want revenge.
When you add this to the general mix of what else can be going on in a world, and war is an inevitability in any world with PCs or with a story going on. If it isn't one kingdom wanting to seize an asset of another, it'll be someone looking to start fighting to climb the ladder. If it isn't those things, it will be someone looking to defend what they have.
Where in your game is there opportunity for chaos to be sewn? In what actions of the PCs is there a chance for someone to feel slighted? What can be done to throw things into chaos? Using the actions of the PCs is a great way to make them feel involved, or even the cause of the chaos.
Work it around. What would someone do to leverage the PCs actions for their own? How can you twist things against them? How can you plunge the world into madness? And from that, who hopes to climb out?