Kill the rich.
One common cause of wars is a surplus of the rich. When you have a surplus of labour, the elites reproduce a lot, and then you have elite overpopulation. The elites then compete for resources, and will start wars.
This leaves your empire unstable. The young won't care about the deals you made with the rulers of regions, as they want more stuff and there's not enough stuff to go around. They'll recruit commoners, anti elite commoners will recruit other commoners, and war will start.
As such, you need a way to get rid of surplus rich people. Duels are one way. Dangerous games for prestige are another. Wars against monsters are a third. Find ways to get rid of elite hotheads, and you'll have a more stable empire.
When you have a surplus of men and not enough women around, the men won't be tied down in stable relationships and they'll be eager for any cause to win them some fame and prestige. War is more likely with an imbalanced gender ratio.
As such, you need to find a way to get rid of men if there's a surplus. Frontier towns with a high death rate. Gladiator fights. Wars against rebels for loot. Find ways to expend men if the gender ratio is off before they expend themselves rebelling.
Kill rebels and dissidents.
Some people love revolutions. They have high minded ideas and want to plan to fix the world. They are a problem for you.
As such, make sure you have a place to send them all. Strongly encourage them, but don't force them, to emigrate to a far away land of freedom.
If all the rebels are in one place they'll fight and kill each other to lead the revolution, and you can focus your troops in one place filled with revolutionaries.
Kill poor self esteem.
Nations love to compete and prove their standing and prestige. A lot of wars aren't over security (which you maintain) or resources (which you control) but over their feelings of prestige and satisfaction.
As such, have lots of international competitions and sports and ways to show off. Give the locals some measure of control over funds so they can compete to show off their skill and effectiveness.
Try to also encourage a culture of shame around war. War wastes resources, kills people, and should not be seen as a way to get honour.
People who are on the brink of starvation or devastation are likely to rebel, to ensure they are safe and sound. Make sure you have stable food lines, and stable jobs everywhere so people can earn enough to live.
Any person who wants to get a job should be able to get a job. In some places you can encourage businesses, in some places you can have more state jobs, or welfare. It depends on the place. Make sure everywhere, people can spend most of their time working, people can get ahead, and people aren't starving.