Not to argue about who screwed the pooch but this is a far more complex problem than missile defense---which we do have. The larger problem here is that because of the proximity of Seoul to NK and the limited ability to immediately neutralize a conventional artillery attack there are more than 10 million people at risk within minutes of the beginning of hostilities.
"A pessimist sees the difficulty in every opportunity, an optimist sees the opportunity in every difficulty” ---Sir Winston Churchill
"Political extremism involves two prime ingredients: an excessively simple diagnosis of the world's ills, and a conviction that there are identifiable villains back of it all." ---John W. Gardner
“You can’t go back and change the beginning, but you can start where you are and change the ending.” ---C. S. Lewis