A summit including both Western and Eastern states of Europe was launched in Helsinki, with the Helsinki Final Act being signed in 1975. This treaty recognized all national borders in Europe and outlawed the use of force to change these borders. The Act stated that “the participating states regard as inviolable all one another’s frontiers as well as the frontiers of all states in Europe and therefore they will refrain now and in the future from assaulting these frontiers. The correct answer is E.