There's no doubt they would react. With several thousand artillery pieces within range of Seoul I think it would not be to surprising if they started lobbing shells over the DMZ.
The US or the South is not going to start anything, if anyone does, it will be North Korea,
it has already been established by our current administration we will handle all issues, diplomatically and by talking...and the penalty going forward is sanctions...
The US is not going to go over there using military force while they are just yapping off and testing missiles.
They are not going to do that.
China is not going to allow North Korea to attack the US, it is not even an option or realistic. Russia probably only puts up with them.
North Korea does not have any real allies when you really think about it.
North Korea is all big talk or that is their way to get what they want, they create tensions in order bring attention to themselves trying to be a world player,
(since their whole focus in that country is military. the rest in that country? oh well..sad)
however that was with the current leader(using tensions/manipulation), the successor is who they need to be concerned with, his son, or whoever it is and what his plans are going to be and how he makes decisions, he may be worse, if that is even possible.. (instead of all talk, he might try to actually do something really stupid). instead of just being a manipulator to get what they want, who knows going forward...hopefully the successor is a little level headed...at least one can hope.
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only way there would be US involvements using force is simply,
if North Korea actually attacked anyone first and if it would be anyone.. it would probably be the South, if they carried it through the South would annihilate them. North Korea would be completely out of the picture after the fact... having a US presence in the South and how the rest of the world would react is the real question.