Once upon a time in a palace by the sea, there lived a princess. The palace, and all the city's residences, lay on a hill, beyond the reach of the worst the sea could offer, but close enough that the same walls that protected the palace also protected the port and so the kingdom thought itself immune from siege. If an army came they could close their gates an count on the sea to sustain them. If a navy came it would surely lose, for the kingdom had the greatest navy in the known world.
Thus the princess grew up with a sense of safety in a time of peace. She learned the arts of diplomacy and negotiation. She learned to negotiate for grain without revealing how badly it was needed, thus preventing the other side from gaining leverage, and she learned how to tell when revealing one's need might result in a lower price. She learned these things though they were never likely to be necessary, because they are what a princess must know.
She learned to take the pulse of public opinion and public health, she learned to move the people with speeches and shift conversations. She learned that her birth had placed the needs of the kingdom above her own and that whatever power or luxury she enjoyed was either to allow her to serve the people or to compensate her for that service. A service she had never been asked if she was willing to preform, but would none the less.
She learned the arts of self defense. Close quarters combat should an assassin sneak into her room. The difficult art of defending oneself while surrounded by panicked innocents she must endeavor to protect, should an assassin attack her in a crowd. The way to turn her knowledge of the land and architecture to her advantage, should a fight range.
She learned philosophy, for it was expected of royalty. She learned theology of many shades, for it was expected that she not only know of the kingdom's gods, but also of those of potential allies, enemies, and trading partners. She learned law, because it would be hers to uphold. She learned mathematics because without it she could not be sure of being equitable. She learned to host a dinner and multiple styles of dance.
She learned and grew in peace and stability until she had become an intelligent and beautiful woman. At which point she learned to say no to many, many suitors. Sometimes politely, sometimes without even words, sometimes harshly, sometimes loudly, sometimes with armed guards.
And then something changed in her life that no one had expected.
It started small, some minor disagreements between members of her kingdom by the sea and a neighboring landlocked kingdom. But the minor disagreements multiplied and escalated. Bloodshed followed.
It became clear that the situation was rapidly moving toward war. A war which the Royalty of neither side wanted. The land kingdom had an army that was second to none, her sea kingdom had a navy of similar distinction. A war would be without foreseeable end. The capitols would be safe, and instead war would take its toll on smaller colonies. Farms and towns. Anywhere too small to have a great wall, or too isolated to survive a siege.
A war would be without winners, but full of losers.
Unfortunately the local lords, barons and dukes and other such leaders, did not see it that way. They saw war as a way to expand their holdings and settle past grudges. On both sides a segment of the nobility was working toward war, and clamoring for it. Power was distributed enough that, even against the royalty's objection, they could have it.
The laws of both kingdoms were hodgepodges collected over a thousand years of whatever seemed best at the time. After frantic searching, double checking, and hasty comparisons, it was clear that there was only one thing that the nobility on both sides would be forced to recognize. Only one thing that could avert the war: Alliance by royal marriage.
The princess was an only child, and one of the children of the landlocked kingdom's king and queen was a son.
Triple and quadruple checking confirmed that it was the only way the law would force the lords into peace. So while the bellicose lords gathered armies in preparation for the coming war, the royalty of the two kingdoms hastily prepared for a wedding.