It is all part of the plan

Obama inherited a pair of wars that he didn't like.  He pledged to get out of Iraq, and then had to backtrack on that promise once he was in office and really came to understand the situation.  As Bruce Bueno de Mesquita explained in The Predictioneer's Game, if American forces leave Iraq then Iraq will quickly become a puppet of Iran.

Letting Iraq become an Iranian puppet would be disastrous--Iraq would start making plans to do the same thing to Saudi Arabia in order to get control of the oil money.  If you dislike Obama then you probably think that he is afraid of taking the blame for that failure.  If you like Obama then you probably think that he recognizes the danger and accepted the lesser of the two evils--leaving the troops in Iraq.  I suspect that both sentiments are mostly true.

Obama is embarrassed and frustrated that he cannot get the troops out of Iraq.  He probably doesn't allow himself the comfort that he didn't really understand the situation when he made the promise during the campaign. He probably regrets the promise, but recognizes that he had to make it in order to get elected.  But I'm sure that he is still looking for a way to finally fulfill that promise.

So he is selling a massive amount of arms to Saudi Arabia in hopes that a real fighting force in Saudi Arabia will be enough to counter-balance Iran and keep Iraq independent after the US forces leave.

It's certainly not a bad idea.  It could work, though I am skeptical.  But he gets points for creativity.

The Saudis must be looking beyond the withdrawal of US forces and preparing for confrontation with Iran + Iraq.  Saudi Arabia has recently been much more interested in paying others to fight their battles for them.  After all, our ongoing war on terror is really just the externalization of the Saudi Arabian civil war.