All else being equal, a nation that spends 10% of it's resourced building machines of war will have a lower standard of living then a nation that's able to spend that 10% on things like education, infrastructure, or even private commercial ventures / R&D. Hence war being destructive to the economy in the sense that you're sinking resources into production that doesn't return value to you in the form of making your life better. Sort of like there's money to be made in boarding up houses after a fire but setting a bunch of fires doesn't help stimulate the economy.
Realistically, however; Things are a lot more complex as always and there are other factors involved like the nation that did spend 10% using their machines of war to take what the nation that didn't made or value returned in the ability to exert your will on other nations ect ect. But it's always a net loss for humanity as a whole....until the aliens come