Which sects of Christianity are support the idea of "Just War"? What are the arguments that they give that support this view? How do they see their views on this issue being related to the Bible?
The "Just War" is a concept very widely accepted by Christians - at least in the sense of acknowledging that there are conditions when Christians are called to fight. Catholics accept it, Anglicans do in principle (though some disagree), so do Lutherans, and many Baptists. They may not all agree on the conditions for fighting a 'just war', but they do generally agree that there are conditions when war might be legitimately fought. In fact it is much easier to list those churches that do not accept it.
Churches that believe it is wrong for a Christian to fight under any circumstances are mainly the historic Peace Churches. These are the Quakers, Brethren and Mennonites (including Amish). There are other specifically pacifist groups also.
If God tells you to go to war than the cause of the war is justified. How you act in that war may not be justified by the instructions you receive from the Lord. There are select few times in the Bible when God said kill them all but besides that its very difficult to "justify" war.
Most Christians who support war do not call it "just" but instead call it "necessary". The idea being to defend oneself from those who would destroy the nation or the nation's allies or the innocent.
I do support the wars in Afghanistan and even the war in Iraq. I am not a warmonger but I do think that it is appropriate to defend ourselves in the case of Afghanistan or defend the oppressed in the case of Iraq. I do not myself have "wars" or "hate" against people I know and believe the acts of war are only valid for duly authorized nations.