NEW HAVEN — The two dozen firefighters who packed into Humphrey’s East Restaurant were celebrating a coming marriage, drinking and jawboning in the boisterous style of large men with risky jobs, but Lt. Ben Vargas spent the evening trying to escape the tension surrounding his presence. During a trip to the bathroom, he found himself facing another man. Without warning, the first punch landed. When Lieutenant Vargas awoke, bloodied and splayed on the grimy floor, he was taken to the hospital. Lieutenant Vargas believes the attack, five years ago, was orchestrated by a black firefighter in retaliation for his having...