From the Comprehensive Rules 4.0:

If the total strength of one side in a skirmish phase is at least double the total strength of the other side, all the characters on the losing side are overwhelmed and killed (regardless of how many wounds or how much vitality each has). When a character is overwhelmed, that character does not take any more wounds — he simply dies. A side with a total strength greater than zero will overwhelm a side whose total strength is zero. If the strength of both sides is zero, the Shadow side wins the skirmish (but does not overwhelm). When the Ring-bearer is overwhelmed, he is killed, regardless of whether he wears the Ring. See also leaving play.