Welcome to this wonderful thread where I ask stuff because I don't know any better and hopefully someone can explain things to me. There are certain things that I never really understood, probably because I lack some background knowledge on these parts, and unfortunately never found anything to read about it.

The first thing I'm not really sure how it works has something to do with space travelling. Since the journey through the warp is how huge armies move from system to system, I've been wondering why it isn't possible for the Black Legion to leave the Eye of Terror through this routes. Being followers of chaos they should be less concerned about the dangers of warp travels and that way bypass the imperial defences right at their doorstep. The only logical conclusion I've come up with is the idea that it's not possible for them to start such an endeavour within the eye of terror and need to get past Cadia in order to enter the route to their destination (Terra for example).

The second thing I've been wondering about has less to do with chaos itself, but more with the defense against it. Basically every planet is supposed to have a Planetary Defense Force which should allow it to fight enemies until reinforcements (Imperial Guard, Adeptus Astartes or whatever's able to intervene). But most books I can think of the top of my head show us that they are in no way able to defend themselves against anything, even less able to engage the enemy until reinforcements arrive. Now obviously this system of defensive forces must have proved itself somehow, maybe it does every single day but no author is focusing on small threats that can be solved by the PDF itself, but I'd honestly like to know why the only image of it I have so far is that of a group of hobby-soldiers who are less reliable than a gretchin as your doctor of choice.

All in all, it would be nice if someone could enlighten me in these aspects of everyday warhammer or share some thoughts with me.
If everything else fails, don't forget: Trust me, I'm a hamster

The Eye is, as I understand it, a warp storm of immense proportions. It is possible to fly through it, but some areas (ie the Cadian Gate) are more stable than others. If you think of it like sea travel, its the difference between trying to sail through a storm, and sail through a storm while in a sheltered area .. both are possible, but one is a hell of a lot more dangerous than the other.

As well as that, there are predators in the warp that would think nothing of tearing a ship apart to get all the tasty treats within, things that dwarf battleships in size. There's a little about that in one of the Night Lords stories, the navigator Octavia dropping out of warp to avoid the ship being eaten.

PDF I'm not sure about. I get the feeling they lie somewhere between a standing army and a civilian part timers, as we would recognise ability levels. The IG frequently indicate their training and equipment is below standard, so if we take the IG as being regular army, that puts the PDF somewhere lower.

That small threats would be quite boring to read about, and possibly boring to write about. At least that's what I assume the authors think. I mean why write about a small Chaos warband, when you can write about a Chaos Armada? Plus which book would sell more?

Another point to consider is look at the power level of the average Marine (Chaos or otherwise) in fiction. They never miss shots. They can go through entire squads of infantry without breaking stride. A single squad of Marines can conquer an entire world. One single Marine spends 20 years on an Ork world, keeping them occupied so defenses can be established elsewhere. Is it any wonder the PDF have such a rough time of it?

// Gonna make my first non-IC post here to help you out:

Chaos, even though they worship the dark gods, still must use gellar fields and all the other precautions which the Imperium uses. No matter who you serve, the warp is dangerous and getting lost/eaten/invaded in it is insanely easy. The Eye of Terror is surrounded by warp storms, making it near impossible to travel out of it on warp based technology, though it has been done on countless occasions, but countless more times those ships have been destroyed while trying to head through. The Cadian Gate is a piece of Necron (Or was it C'tan?) technology, which nullifys the warp for the most part, which is why the storms are not present in the area.

Also, the PDF are a militia which are more of a speedbump than anything else. They aren't really even meant to hold things off, just slow them down if only for a little. They're also there to keep things organized when not under attack, and evacuate people while the real fighters are working. They're not really a line of defense, but more of a moral booster, and an alarm bell.

Finally, HairyHeretic, you're a little wrong here and there. A single marine, unless you're talking about a chapter master like Helbrect or someone powerful like that, cannot take over an entire planet alone. Even then, that'd be an extreme stretch. Space Marines are trained to work in groups, no matter how small, and rely on their battle brothers as much as themselves. This is what I've known, unless it's been proven in one book or another (Which was confirmed to be canon, 40k: Space Marine was actually not canon, but was set in a parallel universe, hence Titus being captain.)

P.S. The IG are actually rather well trained, not including conscripts,and rather well equipped. It's the problem with the universe they are in, where everything else is infinitely more powerful than they are. A lasgun, in lore, is much stronger than any modern day gun, a leman infinitely better than modern day tanks, their training equivalent to that of a U.S. Marine, etc. (Still going off what I've known for the past while)

Well, of course it would be. But simply writing into a new codex that the PDF somewhere once managed to stop an ork threat on their own would legitimate their existance. I don't really expect the interesting stories of Bob, the everyday trooper, who is doing some traffic control or stuff like that, but a simple arguement what they are good for. D33tly's arguement that they are nothing more than the people screaming for help and helping evacuate the civilians is a reasonable answer to that, but I still think it's a bit... cynical to call such people "defense forces".

Yeah, that's what I thought too. Especially in books Space Marines (on both sides) are often what they should be in the lore, a superhuman who's sole purpose is to excel in fighting. As far as I know they don't survive as easily in Tabletop games, where a lucky shot might just kill a Space Marine where he stands. In books however, just like you said, is often portrayed as a one man army, when compared with his enemies. I still recall a book about a Space Marine basically slaughtering a bunch of Dark Eldar completely on his own (Brothers of the Snake, if I'm not wrong). Somehow I can already imagine the plot of a book where the presence of a single Space Marine is sufficient to make a whole Renegade regiment go "Well, it was a nice try guys, but we're on the losing side". Funny enough, Space marines have a harder time when they are used in a balanced game world (like TT).

Thanks for your insight on the warp travel issue. I know that Chaos followers don't have an advantage when it comes to this kind of thing, but I couldn't get my head around the idea that attacking the same bastion over and over would be an easier task than simply trying another warp route out of there. I guess they really lack any other possibility than that.

First thing I want to mention about the Space Marine / PDF issue is that HairyHeretic does have a point. I've heard enough about occasions were a handful of Space Marines were achieving more than a regiment of guardsmen could have after years of war. They are very, very often portrayed as being able to achieve anything on their own, no matter what their opponents are. That's another thing I'm not really into about this glorification of Space Marines since they are mainly thought of the most elite units the Imperium can provide, however, there are so many stories written about the fact that the Imperial Guard is not able to fight on it's own. While I can see that there might be a serious threat where even the Imperial Guard is not enough, just like the PDF the IG is very often reduced to a bunch of cowards who need the Astartes to save them, which honestly doesn't match my image of what the Hammer of the Emperor should be.

The reason I'm so specific about the PDF is that I wasn't able to see a legitimation for their existence, since their "defending skills" are, like you said, nothing more than a speed bump. But in conclusion, with thanks for the inputs I've received so far, their job seems to be something more of a police than of a militia.

Don't get me wrong, the PDF are better than nothing, and actually do some guarding... But from internal threats, rather than external. Let's say an inquisitor found a cultist den in a hive, with 50 or so cultists in in. He'd call in the PDF to take them out probably, if he didn't do it himself, because then he's using an expendable force that isn't needed elsewhere (Unlike IG, who are always needed elsewhere to fight xenos and heretics). They also run the defense batteries, which are a rather heavy speedbump which most writers tend to ignore.

I think that you are forgeting that PDF are made useless for plot reasons ...PDFs are not that bad usually...sometimes they are even better/on par with IG regiments.Cadian guard,Ultramar auxilia etc.

yes but when PDF is capable of manage for example an ork invasion, simply nobody care and nobody know, is too little for be interesting and relevant.
Instead when a DPF is annihilate is the moment to call the ghostbuster the astra militarum or the space marine.

Of course they are useless for plot reasons, but I'd like to see some confirmation that they are actually achieving anything by providing a basic planetary defense. Even if it's not a story, but a little side note in a codex or something background related.