Complex sentence: As the influence of the Palestine Congress and its executive committee began to wane, other groups and leaders became active; in particular, the Supreme Muslim Council (and its president, Haj Amin al-Husseini) became involved in anti-Zionist activities.
Split into simpler sentences: As the influence of the Palestine Congress and its Executive Committee began to wain, other groups and leaders became active. In particular the Supreme Muslim Council and its president, Haj Amin al-Husseini, became involved in anti-Zionist activities.