The war machine needs fuel. It sounds like petroleum products was a major export of the island. Phillip's father worked for Royal Dutch Shell.
The very presence of the German U-boat meant the Germans deemed it important to stop the flow of petroleum from the island to their enemies -- perhaps even to own it for their own war efforts?

Phillip and the people of Curacao were in fear for their own saftey even though the major events of World War 2 were taking place in Europe and the South pacific. This is because in Febuary, 1942 the Lago Oil Refinery in nearby Aruba was attacked, oil tankers were sunk, and a german submarine was sighted in waters near Curacao!