An army transported by one of your fleets can launch an amphibious attack against an enemy force on the coast. Once the attack order has been given and the fleet has rowed to the coastline our army will disembark and attack the enemy. It is not possible for your force to withdraw from battle once this has occurred. Friendly armies aboard nearby ships cannot act as reinforcements. Only armies that have disembarked and are not already engaged in a coastal assault can reinforce them.