So, if North Korea is planning on firing a missile "toward Guam," presumably meaning south and from its mainland, I fail to see how it plans on having that missile's flight path avoid the airspace of both South Korea and Japan. Perhaps its launched from the far southwestern area of NK. Even then its a close call, and would have to take a fairly hard left to go in the direction of Guam.
Now, I understand that countries have a need to test their military capabilities, which may include missiles, but I cannot recall another country in recent time actually firing a missile that crossed through the airspace of its neighbor. Seems to me like that may be interpreted as the good ole' "act of war," that people keep referencing.