One method is for them to get weapons from the CIA. How does the agency get the funds to buy those weapons? They sell drugs, some of it to Americans. Pretty slick. Corruption in high places.
Some terrorists are Americans working for the government and/or corporations. Take 9/11, for instance. The Rockefellers knew about it more than a year before it happened. The so-called "19 hijackers" were here on CIA visas, some trained at military bases, and protected from arrest by FBI higher ups. Like Oswald, they were patsies -- prepped and cultivated by our loving government so they'd have an excuse to increase the profits of the military industrial complex -- a never-ending War on Terror -- creating more terror than they destroy.
For some of the scientific facts behind 9/11, check out the scientists, engineers and architects at http://AE911Truth.org