Hey there akasg,
As @abhishekmpelite pointed out, you don't need internet connection to play the single player campaign and arcade mode. When you install the game, you should have 2 icons that appear on your desktop - Single Player Shortcut and Multiplayer Shortcut. If the game needs you to activate it online, just click activate later. It should have a built-in CD-Key verifier that lets the game install on your drive (provided it is a legal copy of the game). Once you are done with the campaign, you have the arcade mode unlocked and you can play it, as well as replay the campaign on higher difficulty.
Hope this helps,
Captain_WD.