In trying to decide if a war can ever be called just, I go back to Jesus and and I like to focus on the passage in Matthew 26 where Jesus says the all who live by the sword will die by the sword. This passage occurs in the Garden of Gesthemane when Jesus is arrested. It is worth noting that in Hebrew Jesus is named Joshua which is a name he shares with the greatest warrior in Israelite history. Also Matthew is an attempt to tell the Jesus story by using the motif of the Moses story. So the story is that on passover, which is celebration of God's victory over the demi-god Pharaoh, God dies at the hand of a Roman empire headed by the demi-god known as Caesar. Among many other things, I believe that the passion story is a story marked by God's renunciation of violence and armed force as an answer for the world's problems.
The only victor in war is war itself. The war to end all wars led to the good war that led to the cold war and now we find ourselves in the never ending global war on terror. After observing the carnage of the 1st day of the Battle of the Somme, Edmund Blunden remarked that "The war had won, and would go on winning."
One way to look at the cross is to see that Jesus took upon all the evil in the world and the price has been paid. Therefore, there is no need for for any response and the debate on proportion becomes moot. By putting down the sword, God is demonstrating that vengeance will no longer be the source of God's motivation and force the response to all the evil that has been or ever will be done.
This is a weak foreign policy and it may lead us and many others to die but I have to ask, "Don't we believe in the power of the resurrection?"
It is the power of the resurrection that allows all of us, Christian, American, Israeli, and Palestinian to understand that we stand forgiven for the lies we have told, the violence we have committed, the torture we inflicted, and the wars we start.
If all of us are forgiven then all of us can submit to God and to each other. This radical action is how swords are beaten into ploughshares, and the Kingdom of God becomes a more present reality.
Just war theory often strikes me as an attempt to short circuit this process and trade the radical message of Jesus for a vain attempt at respectability.