Ok, but your torpedoes still have to travel to their target. So you have time for defensive measures there. I think you're relying a bit to much on the "before they have a chance to respond". What does that even mean? What does response time look like? How fast can your ship launch a volley of torpedoes? There's just too many variables to make a precise conclusion. I think we'd have to come up with some actual criteria so we could come up with actual conclusions. Aside from the actual logistics, my first response would be to send up satellites to gather as much data on the attackers as I could. What effects does their FTL drives produce. Then I would try affecting their drives through various means. The goal being to knock at least one of their ships out of commission so I could reverse engineer their drive. I'd try and set up a trap to lure these FTL ships in to a small an area as possible. Space is big so a good chance to have an impact would be to bring them in close. It would be a fascinating exercise to contemplate. This is an interesting idea. What would a "border" in space even look like? In the vast distances between star systems, how are you going to detect ships crossing an imaginary plane? How are you going to secure and monitor such a thing? Thinking about it, "borders" have to be inherently based on star system spheres of influence. so your "border" would be more like a bubble rather than an arbitrary plane drawn through space.