From a David Weber post to Baen's Bar Snerkers Only dated July 26, 2009:

Plentary defense considerations

    In answer to the question about planetary defenses, I'm not going to respond in detail. I'll just say this. Given the civilian technology available in the Honorverse, including the amounts of energy routinely manipulated by hyper-capable, multi-megaton freighters, killing a planet is actually a pretty trivial exercise for someone who's willing to die himself in the process. Keeping someone from killing a planet, if that's his sole objective, on the other hand, is a very difficult exercise. There are all manner of safety precautions in place, and all of them can be circumvented one way or another.

    Before anyone gets all bent out of shape about that, let me point out that killing a major American city would be a pretty trivial exercise for anyone who had access to nuclear weapons. You may recall that there's been a bit of mild worry about that very eventuality over the last few years. What makes the "suitcase nuke" scenario hard to execute is that there really aren't all that many weapons of that variety floating around, security on them is actually pretty good, and every one who has the capacity to manufacture them knows that if one of their weapons falls into "the wrong hands" and is used in that sort of terrorist application, there are going to be all sorts of nasty repercussions for the original provider, as well as for the "end user." The US is too big, too powerful, and its assets and capabilities are too diversified to be neutralized by any single-city attack of this nature. By that I mean the United States is still going to be around The Day After, and it's going to have blood in its eye, and it's going to have the capacity to hammer whoever it decides was responsible for the attack. In other words, the bottom line is that what protects American cities from nuclear destruction (even at the hands of fanatics) is ultimately the threat of reprisals. Police forces, intelligence agencies, military forces, etc., etc., can all conspire to make such an attack more difficult, but a tight, cellular organization with access to the weaponry and a tractor-trailer rig could almost certainly pull it off, anyway. I hope that's not going to give anyone bad dreams, but it happens to be the truth.

    The routinely available civilian technology of the Honorverse is far more dangerous than present day nuclear weapons. Ramming a planet with a six-megaton freighter, even at non-relativistic speeds, and even without its impeller wedge would do cataclysmic damage. A single cargo shuttle could become a dinosaur killer, and that's with civilian technology. Load that same cargo shuttle with a couple of dozen ship-killer missiles, get it into even a relatively high planetary orbit, then disintegrate the shuttle with carefully placed explosive charges and use the shipkillers -- launching out of the debris cloud -- as pure kinetic weapons, and the destruction would be difficult to imagine.

    That's why the Eridani Edict is set up the way it is -- to hammer the absolute crap out of anyone who launches such an attack, rather than relying on active or passive defenses to stop such an attack -- and it's why most people really, really don't want to do anything that could be construed as an Eridani Edict violation. Nor does anyone want to get caught trying to "frame" someone else for one.

    One of the consequences of all of this is that planetary defenses, by and large, are oriented against three specific types of threats.

  1. Natural threats to the planets themselves.
    Asteroids, comets, that sort of thing. These are fairly easily dealt with and normally don't require incredibly esoteric detection equipment. Nor, by Honorverse standards, do they require huge amounts of firepower. Icarus is headed for a collision with Earth? Pooh! Send out a couple of light cruisers to deal with the problem.

  2. Suicidal lunatics.
    These are the sorts of threats that careful screening of starship pilots, multi-man crew requirements in critical duty slots, carefully crafted traffic requirements, etc., are designed to at least make difficult. Such precautions really aren't designed to stop mass attacks on planetary surfaces, but rather to intercept/prevent attacks by single suicide-run ships and the like.

  3. Attacks on orbiting planetary infrastructure.
    This is the kind of major naval attack most defenses are actually oriented to stop. Obviously, such defenses would also be effective (one hopes) against a major naval attack on a planetary surface, but the real nature of the threat is recognized as being an attack on the orbital and deep space infrastructure of the star system. Those targets are considered legitimate under both interstellar law and the rules of war, and it is therefore (reasonably, I think) assumed that those are the types of attacks most likely to be launched. And, as I'm sure most of the long time Flies are aware, ways of carrying out those types of attacks have been discussed on the Bar (and in the books) for quite some time.

    The ultimate defense against a major naval attack launched directly against the planet -- that is, an Eridani Edict violation -- is not really to stop the attack at all, because the likelihood of being able to do that is… debatable. The ultimate defense is precisely that which the Eridani Edict stipulates, and what back in the Bad Old Days when everyone was stuck on Old Earth was referred to as "mutually assured destruction." What this means is that super elaborate defenses designed to stop deliberate, sophisticated attacks on the planets themselves are ultimately regarded as both unreliable and not cost effective. One attempts to defend the space around the planet, not the planet itself, against deliberate attack. One attempts to defend the planet itself against "lone-maniac" attacks… for which relatively light defenses will be effective (assuming the [lone]-maniac in question is [detected] in time) and heavy defenses are unnecessary.

    And in the absolute sense, the destruction of a planet like Manticore or sphinx -- or even Old Earth -- is far less catastrophic, relatively speaking, than, say, the destruction of Cedar Rapids, Iowa, would be to the present day human race. (If you happen to live in Cedar Rapids, I'm not casting aspersions on your municipality. I'm just trying to pick a city most people will have heard of but no one is going to compare to, say, the population of New York or Moscow.) In the Honorverse, humanity has distributed its eggs amongst a great many baskets. However catastrophic loss of life on any one planet might be, the casualty totals would be very, very, very small as a percentage of the total human race. I'm not trying to minimize the loss of life or the human cost involved. And I'm certainly not suggesting that the star nation which was attacked would philosophically shrug and say "Oh well, it could have been worse." I'm simply pointing out that while the routine technological capabilities of the Honorverse mean that a single arranged "industrial accident" could wipe out an entire planet, the distribution and those same routine technological capabilities of the Honorverse's human civilization would make that arranged industrial accident survivable for that civilization as a whole.