Complex sentence: He belongs to the second generation in the organization, which joined the movement in the second half of the 1990s, and is credited with reviving the political left on university campuses, after two decades where the Islamists held the upper hand.
Split into simpler sentences: He belongs to the second generation of the organization which joined the movement in the second half of the 1990s. This particular group of activists were credited with reviving the political left on university campuses, after two decades where the Islamists held the upper hand.