As noted on the Civilization Blog, a new hotfix for Civilization: Beyond Earth - Rising Tide has just been released.
This fix addresses the following issue: Occasionally, there were some cases where a player would go to war, but their allies would not join them in the fight.
Now, when a player declares war, all the player’s allies will also declare war on the target. If you are at war, and then you form an alliance with a faction that has not yet met your opponent, your new ally will not join the war automatically (since they don’t have prior diplomatic contact). This change will not apply to existing save games with an active war or alliance, but will apply to all games going forward.
Please update Rising Tide if you are not set to automatically receive updates to the game.