The sad truth behind 90% of alien invasions is that, while very exciting and full of lasers, they so often lead to some casualities. In XCOM: Enemy Unknown (in which the enemies are some of the same ones from the last XCOM game and so are, by definition, 'known'), those casualities are the good men and women at your command. When they shuffle off this mortal coil, they can't shuffle it back on. I've no idea how mortal coils work, but I'm fairly certain they only shuffle one way. Anyway, perhaps this XCOM: Enemy Unknown trailer will shed some light on the subject of death and coils.
If you'd like to shuffle your friends off of their mortal coils, you'll be glad to learn that Firaxis are adding multiplayer. XCOM: Enemy Unknown launches October 12.