Great care was taken when creating Warzone to ensure that cheating is not possible in multi-player games. If you think you see someone that cheated, check out cheaters for instructions on how to prove that they aren't really cheating.
All multi-player Warzone games take place on the Warzone server. This means that it's not possible for players to give themselves additional armies, manipulate attacks, or do anything outside the normal rules of the game.
Further, the Warzone server never shares private information with the client. For example, information about fogged territories is filtered out on the server before it is sent to the player's browser, so trying to inspect the data-stream between the server and client won't reveal anything that would give a player an advantage in a game.
Single-player games take place in the player's browser. This means it is possible for players to hack their client to give themselves additional armies, to spy through the fog, or really to do anything they want. Of course, as the game is single-player, they're really only cheating themselves out of the fun of defeating the AI within the game's normal constraints.
Anyone who posts instructions on how to cheat at single player games on warzone.com (including the wiki) will find their post deleted, and their account perhaps banned.
 Cheat codes
Warzone does not have any built-in cheat codes. Players that want to feel powerful blasting through the AI can easily set up a custom practice game that's weighted in their favour.