A sci-fi collection with a Viking and a U.S. army MP fighting each other on the cover. I lost this in 1967. It was a paperback anthology.
This is the 1962 Ace paperback edition of a 1957 hardcover anthology, The Best from Fantasy and Science Fiction, Sixth Series, edited by Anthony Boucher. The cover by Ed Valigursky is inspired by Poul Anderson's story "The Man Who Came Early". Note the letters MP on the soldier's helmet.