I think it's really stupid to make a blanket statement such as that "wars help the economy".
Have the Iraq and Afghanistan wars helped the US economy? No. It helped certain sectors of the economy, mainly highly privatized institutions. Besides, a company like GM or Ford could have benefited from these wars, and subsequently the people they employ, if the government would have actually contracted them to build more vehicles to be used by the military.
Replace GM or Ford with any other public company and vehicles with any manufacturing good and you'd get the same idea.
But that hasn't been the case, has it? Instead these contracts were farmed out to much smaller, private companies that nobody has heard of and whose only objective is to build military equipment.