"Infiltrating" might be a bit of an exaggeration, because that would mean these groups actually have a well organized nationwide leadership and the capacity for complex planning. I seriously doubt that is the case, the couple white supremacists I have had the misfortune of knowing were borderline mentally ill with major self control issues and could barely hold down a job or stay out of trouble with the law. I have a feeling the military and law enforcement may play to the same desire for power and control that a lot of these people seek, so a lot of them might end up in the military or law enforcement. But I don't think it's an organized plot. Besides here in the United States, you don't need to join the military or law enforcement to get weapons training, pretty much any decent sized town will have a gun range and trainers willing to teach you anything you want to know. There are even advanced training courses that teach tactics, survival and other military type activities.