Reading The League of Extraordinary Gentlemen for this week's podcast has got me thinking. What is it about "historical horror" that works so effectively?
Admittedly, I'm biased. If there is a single event in my life that could be considered my initial exposure to well-crafted horror, it was reading the log of the Demeter's captain from Dracula, given to me to read from an English textbook at age seven. I immediately adored it, and the experience of reading each increasingly desperate log entry has stayed with me for over two decades.
In short, then, I can't be entirely sure if Dracula grabbed me because of its approach, or it grabbed me because I was new to the genre, and therefore its approach was the first to take root in my subconscious.
Either way, though, there are certainly unique advantages to setting a horror story in the past. Isolation is one. My esteemed flatmate once referred to cellphones as "The bane of the modern thriller", and he's right. It's pretty hard these days to find a thriller or horror film that doesn't find itself having to go out of its way to explain why a quick mobile phone call is out of the question. Phones, the internet - Christ, even Twitter - are all means by which the modern world can effortless communicate. Removing those is almost always necessary for good horror (indeed, in some cases, they are turned against us).
Historical horror goes one stage further. Long-distance communication is no longer something taken for granted, and its absence not something to be hypothetically overcome by a recharged battery or a better signal. To us lucky denizens of the 21st Century, a sleepy Victorian hamlet is already unsettling even before the weirdness slithers into town. Everything begins in a state of lonely isolation. Put another way, a modern horror story must do two things, in general: explain how a situation unlike the one we know has developed, and then play that out. Clearly, there have been many such stories that have done that exceptionally well. My point is simply that "It's 1898" is a useful and very effective shorthand. It certainly does the trick in The League...'s second volume, which is very important to the overall success of the book, since Moore tends to be someone who creates a beautiful recreation of an atmosphere, rather than an atmosphere itself. 
The other advantage to historical horror is what we might call "Second Order Inevitability Syndrome" (SOIS). When we watch or read a story, we know that everything that's going to happen is unchangeable, at least until the end of this installment. The dialogue and plot have already been determined. The idea, of course, is that the characters don't know that. For some reason, though, stick in something which the characters themselves know has already been determined, and things get a lot creepier. Again, the Demeter log-book is a prime example, and you can add such diverse examples as the garbled message from the eponymous Event Horizon, the runic inscription in the (admittedly Dracula-tinged) Doctor Who story "The Curse of Fenric" and Captain Keyes unfortunate run-in with the Flood in Halo.
It's not clear to me why this works so well. Indeed, it seems counter-intuitive that a further degree of removal from the action should actually increase tension. Nonetheless, it cleary works, perhaps because (ironically) it offers the characters a view of their own future.
Obviously, this idea doesn't directly translate to historical horror, because the characters don't know that they're in what we consider the past. Still, I think a similar effect might be taking place, a subconscious discomfort with the idea that this "has happened", rather than "is happening". Certainly, it's interesting that (including Dracula) horror that is historical by virtue of when it was created doesn't have the same effect. Not Frankenstein, not The Golem, and often not Lovecraft's work (though he too was a dabbler in SOIS). Of course, this may be external confounding; it may be that the phenomenon is taking place, but is overwhelmed by the difficulty archaic language can have in immersing the modern reader (though that's a problem that affects some readers, myself included, more than others).
Anyway, that's as far as this line of thinking has taken me. If anyone has any other thoughts, you know where the comments section is.
 Another advantage here is that Moore's story can use the comparative primitiveness of Victorian technology as a way to increase the obvious technological superiority of the Martian war machines. Again, it may seem counter-intuitive, but watching the Mars Tripods decimate cavalry regiments and artillery batteries is more effective than seeing them taking on the US military in either of the two film adaptations.