Alien Soldier

In 1860, a red-bearded trapper, Killian Kilkenny, encounters a Star Warrior from the Sky People, transformed into a Navajo Indian. The two engage in a knife fight and fall to the ground. As they struggle, Killian slashes the Indian's arm. A bluish blood spurts into Killian's eyes and open mouth. The Indian stands up. There's a bright flash of blue light, and the Indian's body disappears. A blue-white ribbon of smoke rises skyward.
Killian becomes ill from ingesting the bluish blood and passes out. When he comes to, he finds he's a soldier in the Army's 1st Cavalry. Killian doesn't know how this has happened or how his wounds self-heal. Even more astonishing to him is his ability to morph into any human form; he's almost immortal. Killian learns from an old Indian medicine man that when he ingested the blueish blood, he became a Blue alien, and now, an enemy can only kill with fire.
Killian knows that he has but one chief nemesis, Lupan, an evil Navajo Blue Warrior who calls Killian Taglito Silaada, which is Navajo, for a red-bearded soldier. Over the next one hundred years, Killian uses his new power to fight in all of America's wars. During this time, the CIA tries to discover who this mysterious person is. Over the years, he often encounters Lupan and his evil Navajo alien followers.
However, they cannot kill one another. Finally, during the Vietnam War, they meet on the battlefield. With the help of American and local Montagnard soldiers, Killian subdues Lupan and sets him on fire, sending his evil spirit into the heavens.