Is that a theoretical question or are you asking within some specific context (which you forgot to mention)?
If just theoretical, then there are no in-game cheat codes, but there are two things, which come close:
1. debug mode (for developers)... which you can enable in the configuration file and then use certain keyboard shortcuts to do things like: uncover entire battlescape, kill all aliens, etc.
2. editing a saved game... the save is a human-readable plain text file... and if you don't like having 50 retaliation missions at once, you can just delete them... or you can give yourself 100 billion dollars if you wish, etc.