It Was Based on a Real Incident According to the Wikipedia...
A Japanese pilot named Shigenori did crash on Niihau 7 Dec. Initially the residents treated him cordially. Shigenori did state to a Harada of the attack. Irene Harada and her husband did help him, did not tell others of the attack. The person in charge of the village was Ben as stated in the film. So is the owner of the island. Ben Kanahele did receive the Purple Heart and Legion of Merit for his actions. This became known as the Niihau Incident.
See an article on Historynet which expands on the Wikipedia article. https://www.historynet.com/the-niihau-incident.htm
See the Wikipedia article https://en.wikipedia.org/wiki/Niihau_incident