(1)
the principal world energies 主要な世界勢力
(In the history of human society / for several centuries) / [England] has been / (among the principal world energies)
骨格 = England has been
(3)
hardly sufficient 決して十分ではない
hardly は否定語とみなされる副詞
sufficient condition(必要条件) ⇔ necessary condition(十分条件)