1) As Sam Husseni said, if the U.S. had not committed various atrocities in the ME (including overwhelming support for Israli oppression of the Palestinians, amti-Iraq sanctions that result in the deaths of hundreds of thousands, peraps even millions of civilians to name two) Osama Bin Laden would not have been able to recruit people to stuff envelopes, let
alone commit suicide bombing.
2) Whether the U.S. should do nothing or something in terms of military force, what it is doing now is clearly the wrong thing. The current bombing and sealing of borders will increase the number of startving Afghans by several million. The number of people who hate us enough to want to kill American civilians will be increased, perhaps even multiplied. So far there is not sign that either Taliban leaders, or members of terrorist networks are dying in this -- only civilians and low level soldiers.
(Of course 1 and 2 above are tremendous oversimplifications, omitting not only detail, but nuance. I've left out the U.S. role in creating Bin
Laden, the important implications to Muslims of U.S. troops in Saudi Arabia, the bombing of the medical plant in Sudan ... But I'm looking for a common "left" denominator.)
Both those opposed to and supporting the use of some sort of military force seem to agree that the proper way to pursue this would have been through the international legal system -- the disagreements being over whether (and perhaps what kind) of force should be used to enforce any ruling.
One argument on the use of force seems quite symterical. You can argue for it on the grounds that mere legal action without force behind it is likely to lead to more dead American civilians. You can equally argue AGAINST force on the grounds that it will recruit more to causes such as Bin Ladens and lead to more dead American civilians.
Given that (for it's size) the Taliban government is an extremely murderous one, the same argument could be made either way for Aghani civilians -- that is that attacking or leaving the Taliban alone is likely to kill Afghani civilians.
One point I think weighs strongly against the use. of force: it is one thing to imagine an ideal course that might be pursured by an alternative U.S. in an alternative universe. But it seems that any action taken by the real U.S. in the real world will be aimed at increased U.S. power, with stopping terror networks a poor second. In practice it will be ham-handed, bloody and will lead to more murder in both Afghanistan and the U.S. It does seem that opposing all force by the U.S. may be more likely to slow the murder rate , than supporting a "different war".