Complex sentence: The Irish Republican Army, which then had very few members, was unable to hold off the attackers, and in December 1969 split into the Official IRA and Provisional IRA--with the Provisionals vowing to act as defenders of the area in future.
Split into simpler sentences: The Irish Republican Army, which had very few volunteers, was unable to hold off the attackers. In December 1969, it split into the Official IRA and Provisional IRA--with the Provisionals vowing to defend these areas in future.