If the Ten Commandments say “thou shalt not kill” how can a Christian fight in a war?

Exodus holds the 10 commandments. Specifically, we see: >[Exodus 20:13 (KJV)](http://www.biblegateway.com/passage/?search=Exodus%2020:13&version=KJV)
>Thou shalt not kill. Since the Ten Commandments say “thou shalt not kill” why do many Christians fight in a war? Note this question is not aimed at pacifist denominations, but at those who do support war (at least in some situations).