As long as this is negotiated in game chat, it is allowed. I don't like formal truces, but some people seem to think they are needed in order to win. The higher your rank, the less you will see of this. Good players generally know what is needed to do in order to hold other players in check and don't need to rely on alliances.
From the home page of the site (bolded for emphasis):
Risk all your troops on a daring land grab. Use diplomacy to coordinate a group assault on the game leader. Feel the thrill of victory as you crush your last opponent.
And finally, from the Rule Book:
Rule #2: No secret diplomacy
Any form of diplomatic discussion between opponents must be posted in the game chat in English or in a language that all opponents understand. Diplomacy includes, but is not limited to: proposing truces, negotiating alliances, and coordinating assaults. Secret diplomacy can be hard to prove, but if you suspect it you should leave the players in question appropriate ratings. If you feel certain about players engaging in secret diplomacy consider reporting them in the Cheating & Abuse Reports forum by posting a new topic.