This if for all of the Theatre of War titles. Each game is also discounted by 75%, but the bundle is half the price of purchasing the titles separately.
Other sale items: Frontline Fuel of War $5 (no multiplayer any more though) - http://store.steampowered.com/app/9460?snr=1_8_4__501
Arena Wars 2 for $2 (80% off) - http://store.steampowered.com/app/207730?snr=1_8_4__501
Payday The Heist for $5 (75% off) - http://store.steampowered.com/app/24240/
Payday's Wolfpack DLC is also 75% off for the first time (I had the main game from aprevious sale but was waiting for DLC price to plummet)
Whoa, steam store down?
http://isup.me/store.steampowered.com
EDIT: Back up (15 - 20 min later)