There are several different theories to the answer to your question... Below you will find a few of them. According to the United Stated Department of the Army: www.army.gov "The salute is an act of recognition between military men (and women). Its origin is in the ancient European custom of free men greeting each other by holding up their right hand to show that they had no arms. Prisoners do not salute; they are denied the privaledge." According to CLT Sevigny http://www.geocities.com/CapeCanaveral/Cockpit/4745/Salute.html "The hand salute probably originated in the days of chivalry when knights in mail raised their visors to friends for the purpose of identification. Because of strict adherence to rank, the junior was required to make the first gesture to show they were not challenging the senior. Still, another probability as to the originating of a salute comes from the time when assassinations by dagger were not uncommon. It became the custom to approach each other with raised hand, palm to the front, showing that there was no concealed weapon" According to Wikipedia http://en.wikipedia.org/wiki/Salute The origin of this salute is unknown. One theory is that it came from Roman soldiers' shading their eyes from the intense light that was supposed to shine from the eyes of their superiors (see emission theory of light). Another theory is that it came from when men-at-arms wore armor--a friendly approach would include holding the reins of the horse with the left hand while raising the visor of the helmet with the right, so that one could be recognised. A third theory is that the salute, and the handshake, came from a way of showing that the right hand (the fighting hand) was not concealing a weapon. The naval salute, with the palm downwards, is said to have originated because the palms of naval ratings, particularly deckhands, were often dirty through working with lines. Because it would be insulting to present a dirty palm to an officer, the palm was turned downwards.

Actually I also heard of the knight-version...

In the British Military the salute is in recognition and respect to the rank held not tonecessarily to the individual.

It is definitely the fact that a knight in Armour raised his visor with the back of his hand when meeting another Knight. Their Visors would be held open with the back of their hands during their conversation, aiding visual and audio identification. This also explains the modern day prolonged salute in certain situations.

The reason we salute is to honor each other, acknowledging that we are both or all men and women working to serve and sacrifice. A salute between soldiers is an acknowledgment of that. That's also why we salute when our fellows have fallen.

The way it was explained to me in the Navy, medieval knights would raise their visors with the back of their hands to not only show their faces, but to show that they were not concealing a weapon. It was a sign of respect and trust which carried forward to the modern military.