Complex sentence: Amin al-Husseini, who had led the Palestinian Arabs in the 1936--1939 uprising, had fled Mandatory Palestine in 1937, and spent the war as a guest of the Axis Powers.
Split into simpler sentences: Amin al-Husseini, who was one of the main Palestinian Arabs leaders had fled Mandatory Palestine in 1937 during 1936--1939 uprising. He spent the war as a guest of the Axis Powers.