There is great debate about this but a drill instructor for the Marines will tell you countries who have lost a war salute palm out and countries who have never lost a war salute palm down.

The Hand Salute is quite an interesting thing. Nobody can say for sure what its origin is. Some historians theorize it began in late Roman times when assassinations were numerous and frequent. So as a security measure, any citizen who wanted to see a public official had to approach with his right hand raised to show that he did not hold a weapon. Knights in armor were also known to raise visors with the right hand when meeting a comrade.

It is then believed that this practice gradually became just a way of showing respect. In early American history, showing respect involved removing the hat. Concerned by the wear and tear hats by taking them on and off so many times, the motion was modified to just touching the hat, and since then became the Hand Salute of today.

In British history, in the early 1800s, the Coldstream Guards amended the British military salute custom of tipping the hat. They were instructed to clap their hands to their hats and bow as they pass by. Like the Americans, this was quickly adopted because of the wear and tear on the hats by constant removal and replacing. By the mid 19th Century, the salute had evolved further with the open hand, palm to the front, as has remained the case to this day.

A lot of historians chiefly believe that the U.S. Military salute was likely influenced more by the British Navy. The British Naval salute differs from the "Open Hand" British Army Salute. The British Naval Salute has the palm of the hand facing down towards the shoulder. This goes back to the days of sailing ships, when tar and pitch were used to seal the timber from the seawater. To protect their hands, the officers wore white gloves. Since it was considered most undignified to present a dirty palm in the salute, the enlisted- who were not allowed gloves- saluted so that the hand was held/turned at 90 degrees.