I've looked at the headlines and as great-a-title as it makes, it was the media mis-quoting a North Korean diplomat of some high rank.
He actually said that by putting 'Kim Jong Un on it's list of sanctioned individuals, America had effectively declared war on North Korea'.
This means nothing. It would be like Boris Johnson saying that by having beaten us in the world cup Iceland have effectively declared war on us.
Sadly there was no real declaration.