Recently PCG posted an anonymously written article which contains the following section.
Hamas agreed to be part of the Palestine Liberation Organization (PLO). The PLO is a political organization that claims to be the only legitimate representative of the Palestinian people. The PLO formed shortly after the Six-Day War. It engaged in guerrilla warfare against Israel for over 30 years before it entered into peace negotiations with them in the 1990s. (Will Hamas and Fatah Form a Unity Government?, February 8, 2021.)