Picking up on David's wise recommendation to read the Odom column (see links immediately below), here's one particularly salient point from it. It really sums up everything: "Why are so many members of Congress swallowing the claim that prolonging the war is now supposed to prevent precisely what starting the war inexorably and predictably caused?" Here Odom is talking about the influence of Iran in Iraq.

To me, even more critical is this short paragraph ...

1) We must continue the war to prevent the terrible aftermath that will occur if our forces are withdrawn soon. Reflect on the double-think of this formulation. We are now fighting to prevent what our invasion made inevitable! Undoubtedly we will leave a mess -- the mess we created, which has become worse each year we have remained. Lawmakers gravely proclaim their opposition to the war, but in the next breath express fear that quitting it will leave a blood bath, a civil war, a terrorist haven, a "failed state," or some other horror. But this "aftermath" is already upon us; a prolonged U.S. occupation cannot prevent what already exists.

To me this sums up everything.