Not so - the U.S. hadn't known domestic, large-scale ARAB terrorism up to that point, but terrorism per se has a long and grim history here, from the medicine-bottle poisoners through the Unabomber, various multiple shootings, random serial murders such as Son of Sam, Japanese balloons with bombs attached, Black Hand overspills from Europe, KKK night riders, various incidents by both sides in the War Between the States, not to mention a host of delights during the Vietnam War including (but not limited to) the Weather Underground, the Symbionese Liberation Army, etc, etc...no, it's nothing new to us. Nor is violent response. What's new is that our violent response is hitting home in the Arab nations, and that it is they, and not us, who are going to have to accommodate this wonderful new world that they helped create.
Don't forget WTC1, ten years ago. Personally, I'd throw Beirut, the Tehran embassy, the USS Cole, the African embassies, Khobar towers and an assortment of kidnappings and hijackings targeting Americans over the years.
Those were American's dieing. It's not that we didn't "know" Arab terrorism, we just turned a blind eye to it. Now we're reacting, the the Saudi's chagrin.