Sleeping Beauty

After many childless years, King Stefan and Queen Leah happily welcome the birth of their daughter, Princess Aurora. They proclaim a holiday for their subjects to pay homage to the princess. At the gathering for her christening she is betrothed to Prince Phillip, the young son of King Hubert, Stefan's friend, so that their kingdoms will always be united.
Among the guests are three good fairies: Flora, Fauna, and Merryweather, who have come to bless the child with gifts. Flora and Fauna give their blessings (beauty and song, respectively). Just before Merryweather can speak, the evil fairy Maleficent appears. Angered upon not being invited to the christening, Maleficent curses the princess, proclaiming that before the sun sets on her sixteenth birthday, she will prick her finger on the spindle of an enchanted spinning wheel and die. After Maleficent leaves, Merryweather uses her blessing to weaken the curse so that Aurora instead will fall into a deep sleep from which she can only be awakened by true love's first kiss. King Stefan, still fearful for his daughter's life, orders all spinning wheels in the kingdom to be burned. The fairies do not believe that will be enough to keep Aurora safe, and so they take Aurora away to a woodcutter's cottage in the forest, living as mortals, until the day of her sixteenth birthday.
Years later, Aurora, renamed Briar Rose, has grown up into a beautiful young maiden. On the day of her sixteenth birthday, the three fairies ask Rose to gather berries in the forest so they can prepare a surprise party for her. Meanwhile, Maleficent, in frustration, has her raven Diablo search for Aurora after her bumbling demon soldiers fail to find her. In the forest, Rose's beautiful singing voice attracts the attention of Prince Phillip, now a handsome young man. They instantly fall in love, unaware of being betrothed many years ago. Rose asks Phillip to come to the cottage in the glen that evening to meet her, without telling each other's names.
Having difficulty sewing together a ball gown and preparing a birthday cake for Rose, the fairies resort to magic. The magic with puffs exiting the chimney of the cottage, attracts the attention of Diablo. When Rose arrives, the fairies tell her the truth about her royal heritage, and that she cannot see her newfound love stranger. Heartbroken, Rose leaves the room. Overhearing this, Diablo departs to inform Maleficent. At the same time, Phillip tells his father of a peasant girl whom he met and wishes to marry in spite of his prearranged marriage to Aurora. King Hubert fails to convince him otherwise, leaving him in equal disappointment.
The fairies take Aurora back to the castle that evening. Maleficent lures Aurora away from the fairies and tricks the princess into pricking her finger on the spinning wheel, completing the curse, and falls into an eternal sleep. The fairies put Aurora on a bed in the highest tower and cast a gentle spell on everyone in the castle, causing them all to fall asleep until the spell is broken. From King Hubert's conversation with King Stefan, Flora realizes that Phillip is the stranger whom Aurora has fallen in love with. However, he has been ambushed and kidnapped by Maleficent and her minions at the cottage. They take him to Maleficent's castle on Forbidden Mountain and imprison him in the dungeon. Maleficent shows Phillip that the peasant girl and the now peacefully sleeping Aurora are one and the same. She plans to keep him locked away until he is an old man on the verge of death, then release him to meet his love, who will not have aged a single day.
After Maleficent returns to her tower, the fairies arrive at the castle, where they narrowly avoid being spotted. Luckily, they find and release Phillip, arming him with the Sword of Truth and the Shield of Virtue. The fairies and Phillip then proceed to escape on his horse Samson. In the process, Merryweather also turns Diablo to stone, but the cries alert Maleficent to the prince's escape. As Phillip and the fairies make their way toward King Stefan's castle, Maleficent tries to stop him with a series of lightning bolts, and even conjuring a forest of thorns to surround the castle, but all her attempts fail. She then teleports herself to the castle gate and transforms into a gigantic dragon to battle the prince. The battle moves onto a cliff, where a blast of Maleficent's flame causes Phillip to lose his shield. Just as it looks as though Maleficent is about to destroy him, the three fairies fly to Phillip's aid. Blessing it with all their magic, Phillip throws the sword directly into Maleficent's heart. Mortally wounded, Maleficent collapses over the cliff.
Now that Maleficent has been destroyed, the forest of thorns disappears. In the highest tower he finds his true love, still under the sleeping spell. He kisses Aurora and she awakens, finally breaking the curse and waking up everyone in the palace. The royal couple descends to the ballroom, where Aurora is happily reunited with her overjoyed parents, despite King Hubert's confusion. As Aurora dances with Phillip, Flora and Merryweather resume their argument over the color of Aurora's dress, changing it to pink and blue and back again. Aurora and Phillip live happily ever after.

Question: Why is Princess Aurora betrothed to  Prince Philip?
Answer: to keep the kingdoms united