In order to install/play you will need an internet connection.  I would believe upon initial install you would need to complete the updates to play even single player.  Every once in awhile there is also an update/Steam version check which will disable you from playing online if you HAVE NOT synced your Steam cache, meaning you have to have an internet connection and at least login ONCE every so often - into single player WHILE you are online to update your activation and allow to play while you are in Steam in offline mode... I'm unsure of the frequency but if say you don't login or play for a month and try and Steam reports a message that 'authentication failed' or 'no game data' - I forget what it says exactly... but still every once in awhile Steam needs to sync to enable offline play with no internet connection ** only once if you will only have internet connection once just to setup the computer - as long as the computer NEVER connects to the internet again after you install mw3 and update steam then login to single player campaign at least once, then it will not know that it needs to resync and Steam/MW3 needs to update ** if you are ALWAYS connected, yet prefer to be offline just playing single player, it will read Steam's/MW3s normal update logs and demand you be connected to resync and is required to play even in offline mode... recommendation: when you are online and plan an extended offline break, say you can't afford internet this month and decide to drop service for example, last thing you do before your service contract ends, open Steam and update all games in your library and the games you want to have access to offline for single player like MW3 single player campaign NEED to be logged into at least once so Steam will verify your data, then you can be offline as long as you want AS LONG AS Steam does not load and catch access to the internet even for a couple minutes... otherwise it may read and resync the latest updates and reset disabling you from playing MW3 offline until you login and Steam verifies that you have access to the single player campaign in offline mode... also it is 100% impossible to just install off the disc and play without having an internet connection in case anyone is wondering, since Steam authentication is required.

In order to install/play you will need an internet connection.  I would believe upon initial install you would need to complete the updates to play even single player.  Every once in awhile there is also an update/Steam version check which will disable you from playing online if you HAVE NOT synced your Steam cache, meaning you have to have an internet connection and at least login ONCE every so often - into single player WHILE you are online to update your activation and allow to play while you are in Steam in offline mode... I'm unsure of the frequency but if say you don't login or play for a month and try and Steam reports a message that 'authentication failed' or 'no game data' - I forget what it says exactly... but still every once in awhile Steam needs to sync to enable offline play with no internet connection ** only once if you will only have internet connection once just to setup the computer - as long as the computer NEVER connects to the internet again after you install mw3 and update steam then login to single player campaign at least once, then it will not know that it needs to resync and Steam/MW3 needs to update ** if you are ALWAYS connected, yet prefer to be offline just playing single player, it will read Steam's/MW3s normal update logs and demand you be connected to resync and is required to play even in offline mode... recommendation: when you are online and plan an extended offline break, say you can't afford internet this month and decide to drop service for example, last thing you do before your service contract ends, open Steam and update all games in your library and the games you want to have access to offline for single player like MW3 single player campaign NEED to be logged into at least once so Steam will verify your data, then you can be offline as long as you want AS LONG AS Steam does not load and catch access to the internet even for a couple minutes... otherwise it may read and resync the latest updates and reset disabling you from playing MW3 offline until you login and Steam verifies that you have access to the single player campaign in offline mode... also it is 100% impossible to just install off the disc and play without having an internet connection in case anyone is wondering, since Steam authentication is required.