THE killing of three U.S. troops and wounding of dozens more by Iran-backed militants is piling political pressure on President Joe Biden to deal a blow directly against Iran, a move he's been reluctant to do out of fear of igniting a broader war. Biden's response options could range anywhere from targeting Iranian forces outside to even inside Iran, or opting for a more cautious retaliatory attack solely against the Iran-backed militants responsible, experts say. American forces in the Middle East have been attacked more than 150 times by Iran-backed forces in Iraq, Syria, Jordan and off the coast of…