I'm writing a fictional story and basically that's the case of one of the kingdoms there: conquering others to become more powerful and fight another nation. I'm asking this question to get some historical evidence if that could work.
I'll try to briefly explain the circumstances:
There is a group of 6 small kingdoms (in a European-ish medieval setting) that share a lot of their culture, since they were one single country in the past. In my story, a powerful foreign nation (let's call it P) is becoming too dangerous and has the potential to beat all these kingdoms in a war (and go even further).
The ruler of one of these kingdoms, who is a woman, thinks that the only way to have a chance against P is using the forces of all the 6 kingdoms together. Since she knows all the other rulers and knows that they probably won't work effectively as a group or accept to be lead by a female, her decision is to conquer all the neighbors and become a central ruler of them all.
Of course, making war to all of these countries would take a lot of resources, time, and result in many human losses. However, she prefers to play clever as much as possible, probably making use of force at first against the weaker kingdoms and later patriotism (mentioning the greater nation they once were) and intimidation to get the support of the other ones. Whenever possible, she will avoid battles, since the idea is to have more soldiers in the end.
I also assume that currently P is "busy" with other conflicts and so the woman will have some "extra time" to conclude this plan of unifying the nation.
So, my question is: were there similar situations in real History (even if they were not in Middle Ages)?