Since world war 2 the most important political issue in arab countries has been the ?

The conflict over the establishment of the state of Israel.

Immediately after Israel declared itself an independent nation (free of British mandate control), a coalition of Arab states attacked, in 1948.  Another war over the Suez Canal zone followed in 1956 -- though that was more a direct matter between Egypt and Israel (as well as Britain and France).  Further wars between Israel and Arab neighboring states occurred in 1967 (the Six Day War) and in 1973 (the Ramadan or Yom Kippur War).