North Korea has gunned down a ship of US supported South Korea , but US is not taking any action on North Korea. why ?
if something like this was done by Iran or Iraq etc. immediately US will put sanctions or attack that country
North Korea has nukes and an army that is ready to take losses. Iraq and Iran mainly don't have nukes. The plutonium laced warheads r the main reason. Also- The KPA (Korean Peoples Army) already has faced Douglas McArthur in the 1950s, so the North Koreans arean't getting that "shock and awe" effect, which US relies on 4 success.
The US combat doctrine only works if the adversary gets overwhelmed and awed by the sheer firepower of the US Armed Forces arsenal. But if the US armed force arsenal and all that technology doesn't deter an armed force, and that particular armed force takes losses and continues facing the US- the US combat doctrine flops like a deck of cards.
North Korea is not a soft target like Iran and Iraq.