After Mustafa Barzani's death in 1979, his son Masoud Barzani took his position. As tension increased between KDP and PUK, most Peshmerga fought to keep a region under their own party's control while also fighting off Iraqi Army incursions. Following the First Persian Gulf War, Iraqi Kurdistan saw the Kurdish Civil War between the two major parties, the KDP and the PUK, and Peshmerga forces were used to fight each other. The civil war officially ended in September 1998 when Barzani and Talabani signed the Washington Agreement establishing a formal peace treaty. In the agreement, the parties agreed to share revenue and power, deny the use of northern Iraq to the Kurdistan Workers' Party (PKK), and not allow Iraqi troops into the Kurdish regions. By then, around 5,000 had been killed on both sides, and many more had been evicted for being on the wrong side. In the years after, tension remained high, but both parties moved towards each other, and in 2003 they both took part in the overthrowing of the Baathist regime as part of the Iraq War. Unlike other militia forces, the Peshmerga were never prohibited by Iraqi law.In 2014, the Peshmerga withdrew from the Nineveh Plains which was said by the locals as being a contributing factor of the quick ISIS victory in the invasion, and the widespread massacre of Yazidis, who were rendered defenseless.