Complex sentence: Coordination in foreign policy had taken place since the beginning of the 1970s under the name of European Political Cooperation (EPC), which had been written into the treaties by the Single European Act but not as a part of the EEC.
Split into simpler sentences: Coordination in foreign policy had taken place since the beginning of the 1970s under the auspices European Political Cooperation (EPC). EPC had been written into the treaties by the Single European Act but not as a part of the EEC.