Global thermonuclear war is a game type that hasn't been released.
Hackers on PC found it in the MW2 files and found out what it was.
In this game mode there is a Nuke (looks like a bombsite in S&

etc.) in the middle of the map and you and your team need to capture it.
The capturing itself takes about 1 min. if i am correct.
If there are more people from your team around the nuke the capturing goes faster (just like domination flags)
The team that captures the nuke, launches it and wins the game.