|I used to play 40k, don't now but still remember most of it and left only about a year and a half ago. Anyway, if nations of today unified to face a common threat, and that threat turned out to be from any one of the 40k factions, then how might we cope in the following scenarios:
1) Defending Earth versus a small to medium sized (in 40k terms) attack by any facttion, the attackers are isolated from their main base by whatever mechanism you want, Warp crap or whatever. So beyond what the enemy take with them which is mainly transports and maybe a few escort class ships, that's it.
2)Fighting over an imaginary unihabited island, except for island read planet. Assume we can send our guys their via some way which dosen't bring fleet engagements into this, as we don't have 5km long crusisers.
3) Our capacity to steal technology and develop a fleet and combined arms to take the fight to them across the 40k Galaxy. Assume our Earth is effectively hidden and shielded from the enemy to start by some astrophysical thing which dosen't hurt us. But this is not insurmountable if the enemy get lots of specific intel.
If you're a military genius but don't know anything about 40k google and wikipedia are your friends. My view is that if standard issue military run the show, we get it handed to us by everyone. If we exploit every possible human resource and have geniuses running things, we probably still get it handed to us by 'nids, Necrons and Marines of both types. Sure some of you will disagree so it'll be great to hear replies.