The scourge of “terrorism” — for today’s purposes, let’s take the word to mean attacks on civilians perpetrated by individuals or by small, stateless groups — is a part of human existence going back as far as you want to look. If a person has what he or she views as a grievance, then attacking people who are loosely connected to that grievance, in order to kill and maim some of them and frighten the rest, is obviously one of the options, immoral and hideous as it may be. There’s nothing modern about the strategy of terror.
What’s new about terrorism in the modern world is science. Science, via the technology that it makes possible, is a great multiplier. It allows an individual, or a small group, to exploit power inherent in nature, turning a task that no human could perform, or that would take a cast of thousands, into something that can be done with ease by a few people, or even just one. Of course this multiplied power has many benefits for us as individuals and for society as a whole; think of trains, tunnel-boring machines, skyscraper cranes, snow-blowers, pneumatic drills, aircraft engines, power plants, and on and on. But it also poses many risks and challenges that we have to face, as individuals and as a global civilization.