In 1942, German forces invaded Holland and soon defeated the Dutch army. The Dutch West Indies, which were governed by Holland and included Curacao, had vital oil installations. (Remember that Phillip's father worked for one of the oil companies.) Both the Nazis and the Allies were interested in controlling the oil supplies. Shortly after Germany invaded Holland, Allied troops landed on Curacao to protect those facilities from being taken over by the Nazis. Also, even though Holland had been defeated, the Dutch navy was still active in the Indies. That is why German ships and U-boats were attacking and why people like Phillip and his mother tried to escape the islands.
You might be interested in the fact that Anne Frank lived in Holland and was hiding in the attic at the same time the fictional Phillip was shipwrecked on the cay.
They rather not let any army have oil to run the engines so they torpedoe all the oil tankers so it doesn't get to America for them to use in order to win the war