The fact is, the only way to keep the Kim regime from violating UN sanctions would be a stringent naval blockade. While a full-on blockade would require a Security Council resolution, it would be possible for the U.S. to immediately start putting in place the rudiments of a comprehensive inspection regime on the high seas, which could be easily adapted over time as more allies, partners and ultimately geopolitical competitors like China and Russia can be persuaded to sign on. Indeed, the Trump administration has already been thinking along these lines.Well, we are technically still at war with the NORKS, so it's not an new act of war.
Such a blockade would serve three key purposes: definitively cutting off North Korea’s access to oil imports from the sea; stopping Korean exports, especially textiles and seafood (which are of significant hard currency value to the regime); and ensuring that high-tech machinery and raw materials that might support Kim’s nuclear-weapons and missile programs are not allowed into the Hermit Kingdom.
While China might continue to provide such supplies across the long Chinese-North Korean land border, a naval blockade would increase pressure on Beijing to comply with existing UN sanctions, as any illegal imports would be obvious proof of Chinese violations.
Setting up a naval blockade is a tactical challenge, even for the U.S. North Korea operates commercial and military ports on its east and west coasts of the peninsula, including Nampo on the Bay of Korea and Hungnam on the Sea of Japan. It also has ports in the far northeast of the country on the edge of Russia, which has been one of Kim’s apologists on the world stage. Shutting down the entire flow of goods into and out of North Korea would significantly tax the U.S. Pacific Fleet.
But it wouldn’t be impossible. . .
As the Admrial notes, it would put a real strain on the U.S. fleet, especially without lots of allied help.
CIA World Factbook (from whence came the image above) notes the following "major" seaports:
Ch'ongjin, Haeju, Hungnam (Hamhung), Namp'o, Senbong, Songnim, Sonbong (formerly Unggi), Wonsan