Much of this originated as a comment to a Gay Patriot post. I strongly recommend the post and then the accumulating commentary.
It’s important to note at the outset that government has no money of its own. It only can act as intermediary in the (forced) process of transferring money from one group of private citizens to another.
Now a few remarks on government spending. First, much of the money taken from private individuals and/or their businesses (which are just agencies of private individuals) to pay for government spending is lost to friction, including outright waste (I won’t get into losses to graft. Could the graft be identified, it’d be rooted out. Right?). The waste includes money spent on government middlemen, money spent on government contractors hired to oversee the transfers and transferees, time and money spent schmoozing with lobbyists (both the good ones who are only identifying constituent needs—Congressmen generally fit this bill, for instance—and the bad ones, who are sure to take a taste of their own as the money flows, or to provide some vig in return for future consideration), and so on.
Second, money taken from private individuals to pay for government spending—whether through current taxes, current borrowing (future taxes), or printing money (future inflation)—is money that is not spent in the private sector, or husbanded against future spending in the private sector. Thus, the multiplier effect of government spending, such as it is, is cancelled, if not exceeded, by the known, and fairly large, multiplier effect of private spending that is lost. In many sequences of private spending, for instance, that multiplier effect runs out to 1.7x. The best Keynesian estimate for the government spending multiplier is 1.5x.
Third, those husbanded funds generally are deposited in financial institutions as savings, and these deposits then are loaned to borrowers: these deposits are the primary source of loanable funds. Taxes, then, reduce the amount of private money available for saving and so reduce the amount of lending that can occur. This effect isn’t enough to cause a credit crunch like the one coming out of the Panic of 2008, but it hasn’t helped. Further, that future inflation from printing to support excessive public borrowing devalues the funds that are saved.
There’s more. As government debt grows, more private funds become husbanded, not against future spending goals in the private sector, but against those future taxes. And even more borrowing is encouraged by that inflation (in the period before the recession that is the inevitable result of that high inflation)—in both the private and government sectors—since the repayment will be with inflation-devalued dollars. Then that inevitable recession hits, and payback becomes, as they say, a bitch.
In sum, government spending isn’t zero-sum, it’s negative sum.
Update: clarified the second point.