Complex sentence: At the beginning of the 21st century, antisemitism found new sources from those of leftist views and from the identification of a significant proportion of the Muslim immigrant population with the Palestinian cause on the one hand and with radical Islamism on the other.
Split into simpler sentences: At the beginning of the 21st century, antisemitism was fed from such sources as those of leftist views and support for Palestine. In addition, a significant proportion of the Muslim immigrant population identified with the Palestinian cause on the one hand and with radical Islamism on the other.