A FBI report, reportedly suppressed, made the case that the greatest threat to the U.S. is not from abroad, violent Islamic activists in particular, but right0wing extremists. With legal, ready and open access to a wealth of firearms, these groups and individuals sympathetic to them have had a lot of time arm themselves and have been making their presence known in Charlottesville, Virginia, Portland, Oregon, the armed "Liberate" invasion - for want of a better word - of state legislatures to protest pandemic lockdowns.
Some see them as a harmless threat that could be swept away quickly by the authorities. I am not too sure this will happen. One clear demonstration of this was the recent shooting of protesters by one of these extremists, who was allowed to walk, fully armed, through armed police ranks. It is highly-likely that these militia will be adjunct forces for imposition of "law & order".
The other factor is veterans of which the U.S. has a huge number of. The term veteran does not necessarily mean older soldiers who have done their 20 years or more of service and have retired with a pension, but anyone who has enlisted in the military and has been discharged. It is said that right-wine extremists have recommended their sympathizers join the military for training, and perhaps access to weapons. An unknown number of these extremists could be active in armed militia groups and provide training.