If terrorists didn't kill thousands of Americans on 9/11, those wars don't happen, so you'd have to say 9/11 started it. Ironically, the world had less combat deaths worldwide than ever during those two wars (although this does a terrible job of capturing deaths caused by war, but not directly from combat): https://ourworldindata.org/war-and-peace
The 90s were relatively peaceful compared to the previous few decades, but we still had the Yugoslav Wars, Israeli-Palestine at times, the Gulf war, Rwanda.