Inasmuch as war involves waste/destruction of productive capital, creation/incurring of long-term liabilities, an increase in the cost of finance, creation of additional risk and uncertainty, the expansion of the state’s share of the economy as a whole and the creation of additional government debt, then war is not good for the economy as a whole.
It is not comparable to a Keynesian Demand Management-type investment into infrastructure (a/k/a spend your way out of recession) programme because it’s not investment into infrastructure/productive capital that raises the capacity of the economy, it’s short-term consumption. It’s more like the Broken Window Parable, except a lot more money is spent on equipping the kid with the stone to break the window in the first place.
War involves shovelling large amounts of government money into the accounts of the military-industrial complex. Most of those corporations are either substantially foreign-owned (which isn’t much good for the UK economy) and/or subsidy junkies (ditto).
As an aside, Clegg and Cameron are ending up imitating George W Bush’s first presidency: coming to power on a we’ll-slash-and-burn-government-spending platform, they’ll end up spending an amount of money on foreign military operations that will make the cost of having an extra social worker in Northampton or an NHS slim-down-fatty advisor in Bangor look truly insignificant.