The solution was for America to create a separate type of banknote just for Hawaii. That way, if Japan did take the state over, the US government could at a moment's notice declare all these special Hawaii notes to be as legal tender as Monopoly money. Leaving Japan unable to spend them at any of the international arms depots they usually patronized. The new bill was called the Hawaii Overprint Note, and it came with brown sploshes and big letters reading "HAWAII."
In the end, this measure ended up being unnecessary (unless there's a large battle our history books/Call of Duty games failed to mention). Still, it was a serious project for a while, requiring that every Hawaiian turn over all their existing money. The government collected $200 million in paper money and were going to take it all back to the mainland, but that seemed hard. So they destroyed it all in Hawaii itself, using such local facilities as a sugar mill and a crematorium. We can't find any photos of this glorious fire. But if you want to see a picture of the US military burning millions of dollars while bodies lie on slabs, we're pretty sure there are a couple hundred political cartoons with that exact scene.