More Troops for Iraq

Military leaders are calling for what seems to me a relatively small number of additional troops for Iraq:

The officers said the exact number of extra troops needed is still being reviewed but estimated it at the equivalent of several battalions, or about 3,000 to 5,000 soldiers

Unlike various other pundits and bloggers (most of whom have not served either) I make no claim to any military expertise whatsoever, so I take these officers at their word for what they need. I remain agnostic on whether we had the correct number of troops, if indeed such a thing can be ascertained. The reasons on both sides are obvious, but the answers aren’t, at least to me.