U.S., North Korea mired in a 'tit-for-tat' game on sanctions

Crossing a red line. Unacceptable. Won't be tolerated. Serious consequences. Those are just a handful of the scolding phrases uttered over the past two decades at every bend on North Korea's road to becoming a nuclear state, from pulling out of the nuclear Non-Proliferation Treaty and expelling inspectors to its more recent advances in weapons and missile technology.