Warrior of Two Worlds by Manly Wade Wellman (1903 - 1986)

"He was the man of two planets, drawn through the blackness of space to save a nation from ruthless invaders. He was Yandro, the Stranger of the Prophecy—and he found that he was destined to fight both sides." Another swashbuckler in space from Manly Wade Wellman. Who could ask for anything more? - Summary by the publisher and phil chenevert