The increasing numbers of Jewish immigrants and land purchases, unchallenged by the British Mandate, angered and radicalized many Arabs. In April 1936, Arabs attacked a Jewish bus, leading to a series of incidents that escalated into a major Arab rebellion. The British were caught by surprise and were unable to prevent the deaths of thousands of Arabs and hundreds of Jews in the revolt. The Haganah protected the Yishuv's settlements while the Irgun, a more radical group, attacked Arab settlements. A coalition of recently formed Arab political parties formed the Arab Higher Committee (AHC). It declared a national strike in support of three basic demands: cessation of Jewish immigration, an end to all further land sales to the Jews, and the establishment of an Arab national government. The Arabs threatened that if the British didn't comply with their demands then they would join the adversaries of the British. This concerned the British for World War II was just beginning and they knew they would need Middle Eastern oil.