The Saiyan Kingdom Lore

Formed a little over one hundred years ago, the Saiyan Kingdom is the offspring of the once-powerful Saiyan Solar Empire. Once thought to be unstoppable, all it took to halt the growth of the violent, expansionist empire was a handful of kind-hearted fighters. One of the strongest queens in Saiyan history, Zucceta, and her trusted ally-turned-successor Habana, were integral in changing the fate of the galaxy. It was thanks to their change in laws that the Saiyan Empire faded. 

Slavery was abolished, conquered planets were relinquished, and the barbaric old ways were crushed. No longer were Saiyan children sent out to weak planets to test their might on the populace, and razing cities had become a thing of the past. 

Fighting was, of course, still an integral part of Saiyan culture. The legend of the Super Saiyan had been turned to reality, so quite a few of Vegeta's inhabitants weren't exactly ready to throw in the towel of increasing their power. The army of the now-downsized Saiyan Kingdom had become more of a defense force, with some of its rowdier or feistier members dipping into the occasional freelance mercenary work to get their fill of combat. 

Were it not for the inclusion of Namekians in Planet Vegeta's ecosystem, the transition to a more peaceful life would surely have taken centuries. It was by pure chance that the city of Maestropolis was constructed in the Saiba Wastes during the Saiyan-Mazoku war, but its creation was instrumental in the Saiyan Kingdom we know today. The Namekians were able to, in a way, show the Saiyans how life without war felt. The Saiyans didn't exactly become pacifist monks, but at the least the Saiyan people weren't craving for war anymore. 

Eventually, Habana ceded the throne and retired. A new king took her place. His reign was uneventful, but peaceful nonetheless. Soon after, his son, King Betroo, replaced him. Up until recently, he led the Saiyans as best he could.

That was before the Arcosians made their attack, of course...