Q. Why did North Korea decide to carry out a second underground nuclear test (the first one was in 2006) now?
A. Joshua Pollack points to the North Korean foreign ministry's April 29 statement warning that such a test would occur if the UN Security Council did not rescind its condemnation of a previous missile launch.
Q. Why has South Korea now decided to become a full member of the U.S.-led Proliferation Security Initiative (PSI)?
A. Not sure. Considering that North Korea has said that such a move would be tantamount to a declaration of war against it by the South, and considering that North Korean statements cannot be dismissed as empty rhetoric (see above), the South Korean move would appear to be somewhat risky -- not because it will lead directly to a North Korean attack (though such a possibility cannot be totally excluded) but because it will ratchet up tensions further.