TOKYO – Massive joint U.S.-South Korea military exercises are a spring ritual on the Korean Peninsula guaranteed to draw a lot of threat-laced venom from Pyongyang. This time, not only are the war games the biggest ever, but the troops now massed south of the Demilitarized Zone have reportedly incorporated a new hypothetical into their training: a “beheading mission” against Kim Jong Un himself. It’s the kind of option military planners tend to consider but almost never use.