A couple thousand of them using bombs/weapons could cause massive amounts of continuous long-term damage. Guerilla warfare, as we've come to find out over the past 50 years, is nearly unwinnable. It would manifest as The Troubles American Edition.
McVeigh, John Brown, Bin Laden, Eric Rudolph, Kaczynski, are all the classic extremist that's willing to do whatever it takes to right what they perceive as an injustice.