Where did the common "militart salute' get its start?

I believe it dates back to a medievel period, where knights would raise the masks of their helmets when approached by another knight, as a way of seeing if the person is an enemy or an allie.

Definitely has roots in medievil times. The Knight raising his visor with his right "weapon" hand to asure an adversary that his intentions were peaceful.

Did you know that the U.S. military is the only one to salute with their palm turned down? The reason being, an upturned palm showing the sodier unarmed, means that army has been conquered in battle. The only army in the world that is as of yet, undefeated in battle is the U.S. Army.

There are many theorized origins. One is from tipping one's hat in respect to another person. Another thought was that in Roman times, the soldiers held their hand up to shield the sun. Read the wiki article (if you trust it).

Knights showing that they had nothing in their right, fighting, hand as they approached one another.