SIOP was a plan though - originally it didn't even have any conditionality:

"During the briefings, Marine Corps commandant David Shoup (the service with the most marginal nuclear responsibilities) saw a chart that showed that the initial attack would kill tens of millions of Chinese. At the closing meeting, General Shoup asked General Power what would happen if Beijing was not fighting; was there an option to leave Chinese targets out of the attack plan? Power was reported to have said that he hoped no one would think of that "because it would really screw up the plan"--that is, the plan was supposed to be executed as a whole. Apparently Shoup then observed that "any plan that kills millions of Chinese when it isn't even their war is not a good plan. This is not the American way."