Drawing "red lines" for North Korea would be ill advised. For one, "red lines" imply a certain automaticity of response without regard for situational factors that cannot be accounted for in advance. Drawing "red lines" also implies that any North Korean action just short of the line would be viewed as tolerable and unlikely to elicit a sharp U.S. response, thus creating an unintended dynamic of signaling that certain North Korean provocations would be acceptable as long as they didn’t cross the "red line." In this sense, a certain degree of strategic ambiguity serves the interest of sobering North Korean behavior.
Rather than publicly articulating a "red line," the better focus of U.S. government attention would be to forge strong internal clarity on what top national interests it must protect on the Korean Peninsula. In the case of North Korea, the United States should concentrate on protecting the U.S. homeland against attack or blackmail of an attack, preventing proliferation of nuclear or missile technology from North Korea, upholding the credibility of alliance commitments, and preventing war. Protecting these top interests should be the focus of U.S. efforts, not defending an arbitrary line and thereby creating expectations of an automatic and overwhelming response if the arbitrary line is crossed.