The massive lesson was that industrial war is an awfully bad idea and hugely uneconomic. It turns out that dashing of to shoot up sand or jungle is now just as terrible. After all, any business which consists of dropping thousands of dollars on someone is sure to eventually fail. That is today's definition of war.
The Europeans had to learn that lesson and the USA keeps forgetting it.
As this delightful item makes so clear, that is exactly why the USA emerged as the only true super power. Everyone else destroyed their own treasure to pay for destroying someone else's treasure. A true race to the bottom.