"She also raised the specter of military action, declaring that the U.S. was prepared to use force if necessary" - Why does the U.S. have to use military action?  South Korea and North Korea are still technically at war so any fighting would first be between those two.  If anybody should launch a preemptive strike it should be South Korea.

The U.S. should get out of the business of telling every other country what to do and especially invading countries that the U.S. thinks is a threat.  The U.S. doesn't even have a battle ready army today - the current troops have mostly seen several tours of duty in Iraq and Afghanistan, are suffering from PTSD, and/or are tired of combat duty.