I know many are worried that if Israel could get caught by surprise that we could too.
First of all, if you are going to attack the US you want it to be massive takedown, you don't want another Pearl Harbor that galvanizes the whole country to fight, no, you want a crippling attack that eliminates us attacking. So let's make sure we are talking of the same thing. 9/11 and even Pearl Harbor did not accomplish that. You would have to organize a surprise attack on the scale of the Normandy invasion.
1. To do that you need to have people behind the lines. You would have to have hundreds of thousands of military men in the country able to cause chaos and confusion. So you don't even have to worry about something like this unless you see hundreds of thousands of military age men pouring into the US. It would be nice if we could end the concern here, but with the open border we do see 2 million got aways that we don't even know who it is, but in addition to that the 6 million we have seen come across are mostly men in their twenties. So then, assuming this is part of an organized invasion, let's consider the next thing.
2. The first attack needs to be "anonymous". You can't let the military know who the attack is from otherwise we could fire our nukes at them. If you take out ten electric power stations you can bring down the entire power grid of the US. That could be done with some C4 and probably twenty teams of 100 men each. You want some redundancy in the event that some teams are stopped. Also, these transformers can take two years to replace. Therefore you would want a country like China to have made them already so that when they roll in, six months later, they can quickly restore order. Granted it is safe to assume there are at least 200,000 military men, sleeper cells in the country already. The reason I say 100 men for each of these is because you would want to take down the local power and create several major diversions on the opposite side of town to clear out the police. Perhaps five groups of ten creating mayhem, blackouts, fires and diversions, while a team of fifty then attacks that power station. So, OK, they could do that. In coordination with that you would want a DOS attack on the 911 system, and attacks on the communication system in the country. They may have malware and other cyber attacks on the system. Also, imagine 200,000 signal jammers all being turned on at the same time.
3. Then you also have 12 suitcase nukes from Russia that are already in the country. If those simultaneously go off in twelve major cities it would create mass pandemonium. You want massive evacuations to shut down the highways. To make it worse you could easily have another 500 men whose job is to crash trucks and shutdown highways. Again, something they could easily do.
4. Then as soon as the lights go out and the nukes go off they could also set off the poseidon torpedos that create a 500 foot high radioactive Tsunamis that will hit the entire US coastline. These things hit the US within five minutes of detonation. That means everyone near the coast like the Pentagon and all our naval bases would have to instantly flee. The president and the entire Washington DC leadership would have to evacuate, again creating all kinds of chaos. Nationwide blackout, communications blackout, highways completely gridlocked, panic, pandemonium, and Tsunamis causing massive destruction where 65% of Americans live. At this point we have no idea who is attacking us and we are only five minutes into the war.
5. Then you have all the nuclear missiles fired at our air force bases and missile sites.
It is doable if and only if the US is not on full alert with our extensive intelligence community. The way to do that is to refocus the intelligence community on 75 million Americans as "Domestic terrorists" and focusing on making the military equitable.
But if anyone was actually planning this we would see Putin meeting with Xi and with Kim Jong Un, you would see Iran working with them, you would see Putin saying he wanted to work with Iran, you would see spy craft coming into the US and gathering intel on our missile sites and other military sites. Granted we have seen all that too.