I'm pretty sure I've relayed this here before...The problem is what will be the effect on Seoul if we take action against NK. That has to be taken into consideration. 35 miles is in artillery range.
When I got to Korea in '04, one of our briefers stated how much tonnage of shells per hour was expected to fall on Seoul if the balloon went up. I can't impress anyone with the number because I don't remember it anymore, but it was a lot. Also stated was the average life expectancy for US service members in "2 ID land", AKA Area 1, AKA the area between Seoul and the DMZ, in the event of war; it was less than a minute.
As an aside, I knew a guy who was assigned to a listening post, which is as forward deployed as you can be. He said when his group of newbies arrived at the location, they were greeted in order over North Korean loud speakers with, "Welcome to Korea, Sergeant (or whatever rank each one was) so-and-so, social security such-and-such, enjoy your stay." Freaky.