1) Your turn: Build a ship in a sea zone where there are enemy ships
2) Ally's turn: If your ally then attacks the enemy ships, your new ship does not participate in the battle
3) Enemy ally's turn: Or if the enemy's ally sends ships to attack your new ship, the original enemy ships do not participate
Here's an example from Greyhawk Wars: projpower11.tsvg 1) Keoland builds a ship in Sea of Ulek, where Scarlet Brotherhood (enemy) has a fleet.
2) Iron League (Keoland's ally) attacks Scarlet Brotherhood's fleet, and Keoland's new ship does not participate in the battle.
Here's an example from WWII revised: alliedboatsnothelping.tsvg 1) Britain builds a ship in the same sea zone where Germany has a ship.
2) America attacks Germany's ship, and Britain's new ship does not participate.
Like I said, I won't be surprised if this is well-known. But it seems odd. Especially the fact that if the enemy's ally attacks your new ship, the original enemy's ships don't participate in the battle.
WAI, only one country may attack at a time, while multiple may defend. Not one of the more common rules (hopefully it is in the rule book somewhere), the game does not help very much to make this rule clear, it does look like all enemy units would take part in the attack.
OK, I suppose that makes it consistent with land battles, where there can only be one attacker at a time. Although it doesn't really come up in land battles the way it does on sea zones where you can place ships in territories occupied by enemy ships.