Complex sentence: Gierek's regime between 1970 and 1980 introduced wide-ranging (but ultimately unsuccessful) government attempts to revitalize the economy during a period when opposition circles were emboldened by the Helsinki Conference processes.
Split into simpler sentences: Gierek's regime between 1970 and 1980 introduced wide-ranging (but ultimately unsuccessful) government reforms to revitalize the economy. During this period the opposition circles were emboldened by the Helsinki Conference processes.