Interesting question. My personal idea of "fair" in combat is a tad different than most people, I think...
First of all, I guess we have to decide what "fair" actually looks like in an actual fight. If I'm facing a thug in a back alley who's holding a brick and intent on battering my cerebellum out with it, "fair" might mean that I'm not going to grab his hair, gouge his eyes, bite, or strike at various tender places below the belt. It might even mean I'm going to do my best to hit him in a way that will cause the minimum of permanent damage.
The problem with that is, he's under no such scruples. In fact, he's going to do anything he wants as long as it ends up with me dead and him walking away with my wallet. If I'm fighting "fair" or "just trying to hurt him", I'm going to be severely handicapped in this particular fight. I'm going to die, plain and simple.
In a life-or-death fight, the fighter has to set aside scruples, opinions on fairness, and concern about his attackers life, and focus on staying alive. If that means kicking the other guy in the groin or stomping on his throat once he's down, that's the way it's gotta be. Nobody likes thinking about it that way, but that kind of mindset will save your life.
But that's just for personal combat. With full-blown war, it's really a different story. I believe that the Bible has specific guidelines for how war should be waged, and it's not always what we would think of as chivalrous. When the Israelites were coming into the Promised Land, God commanded them to slaughter every single inhabitant of the country, as well as the women, children, and livestock. That seems harsh, but He obviously had good reasons--as the Israelites found out when they disregarded His command and eventually suffered from it.
But that was also a special case--I don't think God smiles on the killing of women and children in ordinary warfare. The wholesale slaughtering of innocents that General Sherman perpetrated in the Civil War was an ungodly and sinful waste of precious human life. The only difference between that and the Old Testament story was the motive; God commanded the Israelites to kill their enemies so that they wouldn't be led astray by idol-worshipping foreigners living in their midst. Sherman slaughtered women and children to show the Southerners that "war is hell". Not to mention the fact that the destruction was unleashed against his own people...
Wow, that was long-winded. Long way of saying that in some cases, yes, I do believe all's fair in war.

But I also believe that life, as precious as it is, shouldn't be squandered unless absolutely necessary. In the case of the brick-wielding, meth-saturated thug, it's necessary. If I'm marching my army into a surrendering city, it's very likely not.