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.
As Warzone is a competitive game, it's important that cheating never be possible in multi-player games or it would compromise things like tournaments and ladders.