Yesterday I played the Marshall Attack at my club. My opponent didn't know any of it, but thought that it would be dangerous to accept the e5-pawn, so he simply played d4. I had never seen this move before and when I looked it up afterwards, it wasn't even mentioned in either "Nunn's Chess Openings", "Modern Chess Openings", Emms' "Easy Guide to the Ruy Lopez" or Lalic's "The Marshall Attack".

It would seem it must be bad, but according to Fritz it is simply equal. White gets a position with a symmetrical structure where he is slightly underdeveloped but can catch up and there's not a lot left to play for. Why isn't this covered anywhere? This line takes quite some fun out of the Marshall :-(

Interesting game.

10.d4 seems to be the second most common move at the master level.  A score of 45% wins for White would not seem to merit a ?!  Why do you feel that it should be a bad move?

Well, it leads to a position where white already stands slightly worse, being behind in development, which is not really what he wants from the opening. It seems better to either accept the Marshall pawn or to play an anti-Marshall line earlier on.

It's White's 12th move that gives him a slightly worse position.  Had he played 12. cxd4 he would be slightly better.  The Marshall books don't cover 10.d4  because the position is a typical middle game position that could arise from other variations.    The game position is not a Marshall type position and there are no opening issues.  I don't think 10.d4 has any independant significance.  You simply need to have a basic understanding of chess middlegame theory to proceed from here.