A declaration by Secretary of State Rex W. Tillerson that the United States would consider pre-emptive military action against North Korea raises a question that has dogged American military planners for 20 years: How could this be made to work?

The United States has long threatened force. The sincerity of such threats has always been ambiguous, as they are often meant less to prepare for war than to act as a deterrent to North Korea and a reassurance of the commitment by the United States to South Korea.

But there is a reason that, even as North Korea’s weapons programs have passed red line after red line, the United States has never followed through.

Almost any plan would bring a high risk of unintended escalation to all-out war, analysts believe. It would place millions of South Korean and Japanese civilians in the cross hairs of North Korean weapons with few guaranteed benefits.