Absolutely nothing will happen. North Korea knows that they can't withstand the aftermath of any potential attack on the US. Just for shits and giggles, if North Korea theoretically attacked the west coast of the US and killed hundreds of thousands of citizens, the US would not need to do anything. China would invade North Korea themselves for attacking their largest trading partner, remove the regime operated by Kim, and make the demilitarized zone closely operated by China. Russia would do nothing more than vocally condemn the actions of North Korea if the strike were to happen. Kim Jong- un knows damn well that China will not put up with these actions as Xi Jinping (President of China), already despises North Korea and is trying is best to remove any type of association with them. Also, if China didn't do anything, the US along with the rest of NATO could demolish North Korea within a matter of hours. It is not worthwhile for China to defend North Korea as the initial benefit of it being a buffer between China and the US has proved to be less and less useful. It should also be remembered that North Korea's weapons, including their nukes are heavily unreliable. Even though NK has the power to nuke us, it will not end well for them.
If you need more info on the potential conflict, go here:
https://www.quora.com/Who-would-win-a-hypothetical-war-between-North-Korea-and-the-United-States