A war can have a positive effect on the economy when the primary economic problem is unemployment, resource shortages or excessive government regulation. However, the primary problems faced by the US right now economically are not going to be helped. The primary problems I see with our economy right now are excessive debt, over assimilation into a handful of large companies, and hyper stock volatility. None of these will be helped by a war, and both debt and stock market volatility are liable to get worse.
If we do come out of this fairly quickly in a positive manner the confidence building aspect of 'winning' a war may do us some good. But that's about all I can see that might be good.
Jay