evidence: Periods of tension with North Korea come and go, but the situation on the Korean peninsula right now is the most volatile it has been in five years and it looks likely to get worse. [...] On Monday a North Korean merchant ship crossed the countries' sea border, causing both sides to fire warning shots. South Korea says the incursion was intentional. [...] In the meantime, Washington and Seoul have agreed to strengthen their defence of the Peninsula by holding the military exercises Pyongyang hates so much, and responding to its provocations with force. Following the North's latest round of missile launches and drills, South Korea sent up warplanes and shot artillery of its own.