Tragedy all around: American forces in Iraq came under fierce attack on Tuesday, with as many as 12 marines killed in Ramadi, near Baghdad, and with Shiite militiamen loyal to a rebel cleric stepping up a three-day-old assault in the southern city of Najaf, American officials said. In Falluja, where last week American security contractors were killed and their bodies mutilated, American warplanes fired rockets at houses, and marines drove armored columns into the heart of the city, where they fought block by block to flush out insurgents. Several arrests were made. It was one of the most violent days in Iraq since the fall of Saddam Hussein, with half a dozen cities ignited. One of the biggest questions at day's end was the role of most of the majority Shiites previously thought to be relatively sympathetic to American goals.