The episode of Hey Arnold! where a battle was fought over a pig was based on a real event.

The Pig War was an episode of Hey Arnold! where an historic battle is reenacted, depicting the American Revolutionaries and the British fighting over a pig. As it turns out, this was based on an actual event. In the century following the revolution, there was a conflict of borders on the San Juan Islands. Both American and British citizens settled on the islands and there was relative peace. But then, pig nearly started a war.

In June 1859, an American shot a pig eating in his garden. It turned out the pig was owned by a British man, who demanded more compensation than the American wanted to offer. A conflict involving multiple warships and cannons developed, but ultimately the commanding officers on both sides decided to call it quits. You can read more about it at the source

(Source)