5th Century Rome was incredibly wealthy in local economic terms, but was taken down by bands of nomads who considered themselves lucky to own a wagon.
Some of the tribal groups had a numeric advantage, with migrations that literally stretched across entire countries.
Others had a huge advantage in military skill and tactics - including better technology, with a short bow that could punch through a metal shield at a distance of hundreds of yards.
Conversely when Rome originally started to expand, its only advantage was military. It wasn't any richer than its neighbours, but it knew how to put together and drill an extremely effective citizen army.
Today if terrorists had more imagination and skill than they seem to have, it would take very little money to do huge, even terminal, economic damage to London or one or more major cities in the US. An insignificant fraction of the US military budget would be more than enough.
