5.14  MIGRATE  The MIGRATE action is used to move population from one area to an adjacent area. There is no cost in BPs, but one additional population is lost in the area from which the migration is made (ie. if you migrate one population from A to B, then the population in B will increase by one, and the population of A will decrease by two). The action will fail if you do not have enough population in the area for the number migrated AND the additional loss. You may not make GROW actions in areas into which there has been a migration that turn. Migrations into areas that you do not own are nor normally permitted (even if the area is otherwise unowned). Migrations are not allowed from cities under blockade.
Note that you CAN grow in the area you migrated from, provided you left some population behind, or you can grow BEFORE migrating (it's usual to do the two together, either by grow-and-migrate or migrate-and-grow).
In order to make a migration from an area you must have at least two population (one to move, and one to be lost). It is normally better to have at least three, so that there is still some population left in the area after you've migrated away.
Format [ "MIGRATE" ] [ WHERE FROM ] [ WHERE TO ] [ HOW MANY ]

5.15  BARBARIAN INVASIONS  In Dark Age the only kingdoms that may make barbarian invasions are the Irish, Scots and Picts (the Gaelic kingdoms). In Barbarians at the Gate all kingdoms except the Romans and Persians can use barbarian invasions.
A barbarian invasion is an additional form of attack against an area you do not own. Nothing is actually moved but one population is lost by the attacker. The defender loses one army, fort or population (in that order of preference). Barbarian invasions are not allowed against cities. There is no cost in BPs.
Format [ "INVADE" ] [ WHERE FROM ] [ WHERE TO ] [ NOT USED ]