What was the main cause that made Jews the great enemy of the Muslims?
Muslims don't consider Jews enemies. Muslims believe in their holy book (Torah) and their prophet (Moses; peace be upon him). Muslims consider Jews as "people of the book". There are still many Jews residing and living in many Islamic countries without any discrimination or enmity.
However, Muslims may consider the Israelis as enemies because of the aggression and oppression that they exercise against the Palestinians and because of the massacres that they committed against them to drive them away from their lands in addition to what they doing in the AlAqsa Mosque that may lead to its destruction.