Earth 2160 retail version works In SP without requiring internet connection, unless you want to play MP. Steam version works otherwise.
If you buy the box of the game, most probably, will need to stay connected the same way if you purchase the game on steam.
And you can do the same thing with retail versions of game, without need of installing a third party software like Steam, which also creates additional problems. Search a bit and you find a loads of topics how people have problem with backup games or playing in offline mode.
As many people said before, Valve games just need a one-time activation, and you can activate them everywhere you want as many times as you want, then you can happily play offline as many times you want.