Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: Washington (CNN) North Korea might not be able to reach the U.S. mainland with a nuclear weapon yet, but the political fallout from its claimed hydrogen bomb test is here already. Even before the shockwaves of the 5.1 temblor triggered by Tuesday's test wore off, Hillary Clinton was fighting off a volley of Republican attacks, GOP presidential candidates were talking tough and Donald Trump jumped on a chance to chastise China. The episode was a fresh indication of the challenge that Clinton faces in running for the White House on her record as secretary of state, which makes it impossible for her to escape deficiencies in President Barack Obama's national security legacy.The episode is a fresh indication of the challenge that Clinton faces in running for the White House on her record as secretary of stateWashington has tried punishment and inducements repeatedly with North Korea and has failed to change its calculations 
 Question:He then toughened his stance by including _ in his "axis of evil."

North Korea