At least 20 fighters of the Popular Mobilization Forces, an umbrella group of Iraqi militias with ties to Iran, were killed in a wave of U.S. and Saudi airstrikes. Iraq is home to dozens of armed militias, a legacy of the 2003 U.S.-led invasion of the country and the war against the extremist group Islamic State. Now, some of those militias have once again been drawn into the broader conflict engulfing the Middle East. At least 20 fighters of the Popular Mobilization Forces, or P.M.F., an umbrella group of Iraqi militias with ties to Iran, were killed in a wave of U.S. and Saudi airstrikes across Iraq targeting the group’s headquarters, the P.M.F. said in a statement on Wednesday.