Israel and Iran pulled off rudimentary versions of this attack. There are so many shipping containers going into and out of every country, with typically zero inspections of any kind, that it is not actually all that hard to get thousands or tens of thousands of drones anywhere. Hundreds of millions would be hard, but a first strike in the style of Operation Spiderweb that cripples our military is a real possibility that keeps people at the Pentagon up at night. The obstacle to that is not logistics, just that (hopefully) US intel would catch on before it happens.