I would like to find out the title of a Japanese anime feature film (not a series) that I saw in the early 90s. It was about a feud between the peoples of two planets (one very technologically advanced, the other not). The two planets go to war against each other, and a young man from one of the planets falls in love with a princess from the other planet. I recall a scene where the army of the second planet take off in hot air balloons when they go to war. The only other thing I recall is that there is a dam next to the village where the young man lives, and for some reason he opens the dam gates and floods his own village. Thank you.
I think you are talking about Windaria, described below.
The mountainous kingdom of Paro and the coastal city-state of Itha had been at peace for over a century. They would have remained that way but for the ambitions of Paro's power-hungry king. Paro invaded Itha, despite the love between Jill, Paro's prince, and Ahanas, Itha's princess. Caught between duty and emotion, the star-crossed lovers were forced to fight a war to its bitter conclusion. Isu was a simple farmer from the neutral village of Saki, but he saved Itha from being destroyed by a spy from Paro, and found himself hungering for wealth and glory that the Ithan monarchy could provide. Agents from Paro made him an offer to fulfill his wishes, and he left his wife Marin behind in what would turn into a battlefield between the two kingdoms.
It's not about two planets, but two kingdoms; otherwise, the story is as you described.