Thank goodness national security is in the hands of the diverse.
If someone or country wanted to really cripple the USA with a cyberattack the three areas I expect to get attacked are air traffic control systems, financial systems and the electrical grid, take those three areas down for an extended period of time and the country would resort to anarchy quickly
1st step - small independent businesses
2nd step - hospitals with links to everywhere with a little disruption in health care as a bonus
3rd step - US banking
4th step - the US government itself for the win
Anyone doing this, putting lives in danger, should receive the death penalty when caught.