Not really true. People tend to look at GDP as the measure of an economy but war time economies distort the picture of economic productivity that GDP provides. If I go out and buy a car, I have first of all put money into the economy but then I drive the car and do things like go to work or shop and further contribute to the economy. If the government buys a tank and at worst it is destroyed or at best it goes off to fight a war to expand territory that is of little value in the long run, it has made no further contribution. That value is completely lost once the tank is paid for. Now if the government builds a bridge. Then, like the car, that will increase economic activity like allowing people to travel and go to work. However, bridge, the tank, and the car are all counted toward GDP just the same.

Patrick Boyle has a pretty good run down on youtube of why GDP is a misleading measure when applied to war time economies.