Today was supposed to be the launch day for Xbox Live's first free-to-play game, Happy Wars. However, the free-to-play revolution hasn't taken hold on Xbox yet, because the game isn't there. As of writing, the only Happy Wars content available on the Xbox Live Marketplace is a trailer; the game's entry on the Marketplace as accessed on the Xbox only gives you the option to "rate" it.
"I know the team is aware of the issue and actively working on it," Larry "Major Nelson" Hryb told Joystiq, promising to tweet an update when information is available. In the meantime, you'll have to find another cheerful skirmish on which to take out your frustrations.