Forces that are not large enough to be effective could be destroyed if attacked by a superior force, so it is prudent to merge any fragments together. It is possible to merge two friendly armies or fleets together when neither is at full strength (a full strength force is twenty units in size). Merge by left-clicking on your first one and then right-clicking on the second. The first force will then move over to the second and the two will merge together. However, it is not possible to have more than one general within an army, so you must disband the lesser of the two after transferring his units. If there are twenty units or more in either force the merge panel will appear, so you can choose which units or ships to merge into a single force and which to leave behind.