The Iraqi offensive aimed at driving the Islamic State group out of the northern city of Tikrit has been slowed by booby traps, snipers and suicide bombers, but received a powerful boost last week when a U.S.-led coalition began launching airstrikes in support of the operation. [...] Iraq's beleaguered military -- which crumbled in the face of the IS group's lightning advance last summer -- has relied on Iranian-backed Shiite militias.