Driving economic activity by having the military act as a major consumer is really just running on a treadmill. We need to find an outlet which will lead to increases in economic activity. There is a huge investment hurdle involved, but space exploration and colonization could fit the bill. Once there is a significant human presence in space beyond low earth orbit, there will be tremendous economic growth. The major problem is that hundreds of billions of dollars of investment would be required to jump start it.

I think you're missing my point. It's not that military spending drives manufacturing, service, and R&D industries (although it does), it's that direct military action has often been, and is often used to promote US economic interests. See the military's actions in Latin America in support of United Fruit, military support of right-wing (pro-US and pro-business) dictators everywhere, or the military-political interventions in the Middle East in support of energy interests.

My contention is not simply that making tanks is an economic activity that many people benefit from, but that those tanks are used to assert and defend economic dominance, which has a massive spillover effect for other industries.

And the need for the assertion of force to defend economic dominance in turn cements the need for the military machine and perpetuates the military-industrial complex. We are essentially in agreement. I think it would be better for the US to concentrate on economic dominance through sheer commercial and industrial awesomeness combined with forward-thinking research and exploration.

You missed my point. Sure, space exploration won't cure our economic stagnation in the 1 to 30 years time frame. But it will decide if we are a first, second, or third-rate power in the next 100 years.