China
It would take 100 well trained operatives with intelligence that China has access to to bring down the entire US electric grid (estimates are that China has well over 1,000 already in the US). All they need to do is destroy ten electric substations which are generally not even guarded. It would take special ordinance like plastic explosive, but if you do that and then sit back estimates run as high as 90% of Americans will die as a result (
https://www.powermag.com/expect-death-if-pulse-event-hits-power-grid/). Many of these substations take more than a year to build and many of them are built in China. So China could build these ten substations they plan to take out before the attack planning to come in as a "peace keeping force" six months later and restore electricity and restore order.
Russia
Meanwhile Russia has a poseidon nuclear torpedo (
https://en.wikipedia.org/wiki/Status-6_Oceanic_Multipurpose_System). It can create a 500 meter high radioactive Tsunami. 65% of all Americans live within 5 miles of the coastline. These torpedos could cause cities on our coast (Boston, NYC, Philadelphia, Baltimore, Washington DC, Miami, Houston, San Diego, LA, San Francisco and Seatle, etc) uninhabitable. People who are not killed in the initial attack would then be homeless, jobless and would become refugees. If these city dwellers fleeing the coast were rugged outdoorsmen, fully equipped and well trained they might survive.
Now imagine combining the two attacks so that at the same time the power goes out these tsunamis hit and the refugees are now fleeing into the US at the same time they have lost all power.
I think it is something like 98% of our military in the US uses the domestic power grid, so that would be bad as well. Without electricity pumps won't work and so you lose running water and you lose gasoline and diesel. Also the banks, debit cards and credit cards would not work. This means those fleeing the coast will likely only get 100 or 200 miles before the cars are dead.
This is a country with 390 million firearms. So you can be sure we will have roving gangs that are well armed. They'll survive the first month by robbing and looting. But in this dystopian world the hospitals will be all but useless and who is going to go to work? Men will be defending their families and homes. Sit back for 6 months, let them shoot it out, let those who were not "preppers" starve. Cholera, typhoid and dysentary will kill off more than the roving gangs.