Beginning with our earliest caveman ancestors, men have painted themselves before venturing forth to hunt or go to war. “War-paint” is used to this day, as we see modern warriors on the gridiron painting their faces before entering the field of play; and American fighting men-and-women painting theirs before going on patrol.
The ancient Celts used a indigo blue dye, woad, extracted from a plant of the mustard family, to paint swirls of color across their naked or near-naked bodies. Together with effect caused by spiking their hair with lime, Woad lent the blue-painted Celts a fearsome aspect, terrifying to their civilized enemies like the Greeks and Romans.
(The seeds of this plant have been found in ancient Neolithic cave sites, so it is likely the Celts weren’t the first humans to discover the value of using Woad as a “war paint”.)
The Polynesian people native to New Zealand, the fearsome Maori, took it one step further, by tattooing such symbols permanently on their faces and bodies.
In North America, the Native American peoples donned war paint as well before they went into battle.
Part of the reason is the obvious: the ferocious and frightening aspect this lends to the warrior/hunter. Another reason may lie in religious practice. We know that Roman conquerors, riding on chariots through the streets of Rome in triumph, painted their faces with vermillion to mimic the red-faced war god, Mars (or perhaps Bellona).
Another explanation may be for the same reason modern athletes and some military men rub black paint under their eyes and across their cheekbones: to cut down on glare. Sunlight can reflect off of sweaty cheekbones, causing a distraction at a critical moment. When being charged by an angry bison or by an enemy warrior, one needs to be totally focused, not blinded!
Modern warriors paint their faces for more practical reasons: as part of an effort to camouflage themselves in the colors and patterns of their background. Since at least WWII, American forces have used “cammie” paint (along with camouflage uniform patterns) to help blend into their environment.
Wither for religious, practical, psychological, or merely artistic reasons warriors of all types (from US Marines to NFL players) have always and will continue to paint themselves for war.