Is "The Soldier" by Rupert Brooke an iambic pentameter?

Yes. The basic formula is there but as the previous answerer said, it varies to relieve boredom,or to fit in a word that would be awkward otherwise,but when that happens another 'error ' is made to maintain the symmetry. It isn't easy,as anyone finds out if they try doing it.Shakespeare(or Bacon,if it was really him) wrote in iambic pentameters.Cly style,but hard to do.