How do!
Been seeing the odd comment here and there about people wondering whether their dream army is 'legal'. And t got me wondering if perhaps I have an odd take on the background and setting of the game.
To me, the background isn't some ongoing political drama, and it's not about moving it forward. Instead, as mentioned at the first Horus Heresy Weekender (totally recommended for peeps to attend!), 40k is a Galaxy stuck at one minute to midnight. And the background to this apocalyptic setting? It's mythology. Consider the stories and such that we read, and the histories of the various races. Very, very little of it is known 100% for sure.
Horus Heresy for instance. Pretty much all that is definitely known, in terms of the 40k Galaxy, is that Horus turned to Chaos, took a bunch of Legions with him, lost the war and the Emperor wound up enthroned. That's it. That's as much as anyone can say for definite.
War in Heaven? Very little known. Necrons kinda won, kinda lost. Eldar kinda won, kinda lost. Old Ones? Far as anyone can tell, they definitely lost it.
Orks? Don't care much for history.
Tau? Well who knows. There's the ongoing mystery of the Ethereals, and that's about it.
And that's just the ancient history. In the modern day? Even more legend. The galaxy is a place of deep seated ignorance and half remembered lore. The galaxy is also, frankly a really, really big place. And what with the warp, and the oddities that lie therein, literally anything could happen.
In short, there's little reason to hold completely true to the background. Yes there are some immutable things (The Emperor isn't quite dead, Horus doesn't appear to ever be making a return) but the rest? Let's look at some.
'No way would Orks ally with Space Marines'. In the general run of things, likely not, no. Nothing there you would necessarily consider to be common ground. But....Freebooterz, and Chapter on the very verge of going rogue. Is it really completely inconceivable that in a galaxy as big as ours that a Space Marine Chapter Master has never hired Freebooterz?
Necrons and Tyranids - Mindshackle Scarabs. Dead easy. Or, some kind of Tyranid gribbly that can corrupt Necron engrams? It's possible!
No need to be completely tied to the background. It's a sandbox folks, so go play!