All of the above, plus:

Seoul is built in a river bed;
the dam in in N. Korea, and is
mined.

S. Koreans still have family
feelings for the North, literally.

Assassination Droids might work;
Richard the Lion Heart ended up
paying protection money to the
original Hashasheem because they
left a dagger on his pillow, three
nights running, in spite of all the
guards he had around his tent.