By the same token, you could argue that the government giving money to any individual or entity could stimulate the economy because those dollars will eventually circulate into the broader market. Keynes claimed that during an economic downturn, it's worthwhile to pay somebody to dig a ditch and fill it back in because he'll spend his paycheck and stimulate the economy. The trouble is, no additional wealth is created in the process because you're using the same amount of money taken from person X to pay person Y to dig the ditch and fill it back in. Arguments against Keynesian stimulus in the civil sector apply just as well to military Keynesianism.
Terrorism is an intentional human phenomenon, the rotation of the earth on its axis isn't.
If the war was about fighting Islamic terrorism, why was the largest amount of resources committed to overthrowing a secular dictator in a country which (until after the war) had no Al-Quaeda presence? Meanwhile, Saudi Arabia, the nation most responsible for sponsoring and exporting Islamic extremism abroad, was coddled. As relevant as Iraq was to fighting Islamism and Al Quaeda, the whole enterprise may as well have been about the Earth's rotation.
But paying someone to develop or refine new weapons systems is not the same thing as paying someone to dig a hole and fill it back in.
That kind of work often generates additive innovations.
If the war was about fighting Islamic terrorism
The war in Afghanistan was about terrorism.
The war in Iraq was about overthrowing a dictator who had committed various acts of war against the US that had not been responded to - and, realistically, it was about asserting US power in the center of Asia.
And, of course, it is just silly to refer to Hussein as a secularist.
One of his pet projects was building the world's largest mosque and one of his prize possessions was a Koran he had calligraphers write in his own blood.
On the issue of terrorism, Huessein had been sheltering Abu Nidal for a decade.
He was killed not long before the US invaded Iraq.
The reliably anti-American Robert Fisk alleges that he was killed because he was an American spy trying to falsely connect Hussein with Al-Qaeda.
More likely he was killed because he was the one link Hussein had with Al-Qaeda.