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If both armies are from the attacking side they combine. If one is from the defending side, and the other from the attacking side, the defender's armies combine. If both armies and the defender are all from different nations, one army will attack the defender, then the other army will attack whoever is left from the results of that battle. It is therefore an obvious advantage to go last but there is no way to control (or, really, even predict) which nation will go in which order.