Why a4?

I noticed quite a few people who have played this move against me (and 1. h4 also) and if they use it "properply", it is designed to gain initiative in pressuring the kingside or queenside from move 1.  When I first saw it, I was rather unnerved by it, but as people tried it more and more against me, I found the best way to play against such an opening is seize the center and prepare an assault on the king, which at the best guarantees you a quick checkmate if they are sloppy, or a nice positional advantage if they don't try to fight for control of the center.  Don't get me wrong, getting that pawn to a6 can be no fun for the Black player, so you always want to evaluate the possibilities of that pawn causing trouble for you.  But if you have the situation under control, then play 1...e5 and play it like an Italian Game for White...f2 is critical! :-)

http://www.chess.com/forum/view/chess-openings/1-a4-best-chess-opening2