Lump of labour fallacy

"This fallacy also occurs with variables other than labor. As an example, some critics have suggested that certain arguments justifying American military action in Iraq during the insurgency of 2003–2004 commit this fallacy [1]. An example of such an argument is implied by the statement: It is better for America to fight the terrorists on the streets of Baghdad than in the streets of New York. According to these critics, this statement implicitly assumes that the number of terrorists will remain constant and will eventually diminish as terrorists are killed off by military action in Iraq."

Indeed.

By the way, I love wikipedia.