niamey/dakar chad withdraw hundreds troop neighbor niger help local force fight boko haram islamist militants humanitarian source officials say pull-out past two weeks could weaken region-wide struggle militants kill tens thousands people force many flee trigger humanitarian crisis immediate explanation comment defense officials chad move come month vast central african country complain unexpected u.s. travel ban impose nationals chad warn time order could affect security commitments include involvement u.s.-backed fight boko haram residents say withdrawal already impact niger diffa region see string attack boko haram militants cross base neighbor nigeria ibrahim arimi border village bosso say banditry increase since chadian troop start leave temporarily move another village safety diffa parliamentarian lamido moumouni say residents start complain come rely force perception security lack say telephone peak 2016 attack bosso chad 2,000 troop niger help counter boko haram although security source say fall since boko haram attack chad niger cameroon base northeast nigeria eight-year bid carve islamist caliphate drive millions home 200,000 base diffa little prospect return home thousands camp alongside unfinished highway middle barren savannah resources chad soldier also occupy front-line position peacekeeping force northern mali fall oil revenues price crash 2014 also sap chad appetite expensive regional security commitments analysts say chad withdraw troop fight boko haram niger