Ever since 9/11, the threat of terrorist bombs on U.S. soil has become a major concern, drawing the attention of hordes of federal investigators and journalists. What few Americans remember clearly today is that barely 40 years ago, during the tumultuous 70s, such bombings were more or less routine, carried out by a half-dozen significant groups of underground radicals, from the Symbionese Liberation Army (best known for kidnapping the heiress Patricia Hearst in 1974) to lesser-known outfits like the F.A.L.N., a Puerto Rican independence group that bombed a Wall Street–area restaurant, Fraunces Tavern, killing four people in January 1975. Amazingly, during an 18-month period in 1971 and 1972, the F.B.I. counted more than 1,800 domestic bombings, almost five a day.

By far the best known of the radical underground groups was Weatherman, later known as the Weather Underground, which detonated dozens of bombs across the country from 1970 until it dissolved in late 1976. A splinter faction of the 60s-era protest group Students for a Democratic Society, Weather has been the subject of a dozen books, memoirs, and documentary films; its best-known leaders, Bernardine Dohrn and her husband, Bill Ayers, remain icons on the radical left to this day. Yet despite all the attention, very little has ever been revealed about the group’s internal dynamics, even less about its bombing tactics and strategies, a topic that few Weather alumni, mostly now in their 60s, have ever been eager to discuss publicly.

Partly as a result, Weather’s seven-year bombing campaign has been misunderstood in fundamental ways. To cite just one canard, Weather’s attacks, for much of its life, were the work not of 100 or more underground radicals, as was widely assumed, but of a core group of barely a dozen people; almost all its bombs, in fact, were built by the same capable young man—its bomb guru. Nor, contrary to myth, did Weather’s leaders operate from grinding poverty or ghetto anonymity. In fact, Dohrn and Ayers lived in a beach bungalow in the seaside village of Hermosa Beach, California.

Of far greater significance is the widespread confusion over what Weather set out to do. Its alumni have crafted an image of the group as benign urban guerrillas who never intended to hurt a soul, their only goal to damage symbols of American power, such as empty courthouses and university buildings, a Pentagon bathroom, the U.S. Capitol. This is what Weather eventually became. But it began as something else, a murderous core group that was obliged to soften its tactics only after they proved unsustainable.

After closing S.D.S.’s national headquarters, 100 or so Weathermen began going underground in January 1970. They split into three groups, one each in San Francisco and New York, the third a loose collection of cells spread across Midwestern cities such as Detroit and Pittsburgh. Outside the leadership, there was widespread confusion as to what kinds of actions were authorized. There would be bombings, everyone assumed, but what kind? “There was so much macho talk, you know, like the Panthers: ‘Off the pigs,’ ‘Bomb the military back into the Stone Age,’” recalls Cathy Wilkerson of the New York cell. “But did that mean we were actually going to kill people? I never really knew.” Bill Ayers and others would always insist there were never any plans to harm people. The handful of Weathermen who crossed that line, Ayers claims, were rogues and outliers. This is a myth, pure and simple, designed to obscure what Weatheractually planned. In the middle ranks, it was widely expected that Weathermen would become revolutionary murderers. "My image of what we were going to be was undiluted terrorist action," recalls a Weatherman named Jon Lerner. “I remember talking about putting a bomb on the [Chicago railroad] tracks at rush hour, to blow up people coming home from work. That’s what I was looking forward to.”

In fact, what constituted a legitimate target for a Weatherman bombing was the topic of sensitive discussions among the leadership at their last major public gathering, in Flint, Michigan, in the final days of 1969. It was during these talks, according to an early Weatherman leader, Howard Machtinger, and one other person who was present, that it was agreed they would, in fact, kill people. But not just any people. The people Weatherman intended to kill were policemen. “If your definition of terrorism is you don’t care who gets hurt, we agreed we wouldn’t do that,” recalls Machtinger. “But as to causing damage, or literally killing people, we were prepared to do that.” According to one side of the argument, says Machtinger, “If all Americans were compliant in the war, then everyone is a target. There are no innocents. . . . But we did have a series of discussions about what you could do, and it was agreed that cops were legitimate targets. We didn’t want to do things just around the war. We wanted to be seen targeting racism as well, so police were important.” Military personnel were ruled to be legitimate targets too.