The initial strike on the North Korean Regime could be from a few obvious things:

1. Immense Troop build near the 33rd Parralel
2. North Korean Artillery pointing their guns at the South Korea on key areas of infrastructure power plants, water plants, telecommunication area or centers
3. Another ICBM test by the North Koreans
4. Threats of hitting the US except the language has intent to really hit the US unlike provocative North Korean Propaganda
5. North Korean Patrol Boats seizing commercial ships who are in or near their waters

These 5 points could start an initial strike by the US.

Whom could launch a Initial Strike, I personally think North Koreans would kick off the initial strike caused by a "broken arrow". I say once a ICBM test missile goes past the 1st ring of ICBM test range, I think somehow if their intended target was a coordinate somewhere in the open ocean, something could happen and could land somewhere else. Meaning either someone else got a hold on the missile through hacking, or their was a pre-planned launch coordinate someone forgot to erase but was left in the missile flight plan and that was the intended target.