You need an army to act as the cargo, and you need at least one fleet to act as the transporter, though you can chain convoys for longer distance convoys. It counts as one unit attacking, but convoys can be supported the same way a move can be, though be careful when inputting these orders, it`s easy to muck it up.

An army can be convoyed from a coastal territory by any fleet that is in an adjacent open body of water, including that of another country`s if that country puts the correct orders in for your convoy. The army can then be convoyed to any other coastal territory that the fleet is adjacent to with the same mechanics as a move order, and may also be convoyed along to another fleet in an adjacent body of water for which the logical process essentially continues.

The convoy can be disrupted if a fleet that is participating in the convoy is displaced by superior forces. A convoy can be support moved into the final destination, and the convoying fleets may be support held to reinforce their position.