premise: The army sent in reinforcements and imposed a curfew on the fifth of Hebron still under its control following the transfer of most of the flashpoint West Bank city to Palestinian rule in January, officials said.
  Palestinian police tried ineffectively to prevent protestors from entering the Israeli area during the clashes, but Israeli security officials complained that not enough was done to curb the rioters.
  Palestinian West Bank security chief Jibril Rajub lashed out in return at the 400 settlers who live in army-guarded enclaves amid Hebron's 120,000 Arab residents.
  "This is an organized crime encouraged by the Israeli government," he told reporters following Arafa's funeral.
  "What happened today shows there can be no coexistence between us and Israeli settlers, who are here against our will and against all laws. This is terrorism itself."
   Why were army reinforcements sent in?
hypothesis: Palestinian police tried ineffectively to prevent protestors from entering the Israeli area