I think the motive of the bashan invasion of irvington was to send a few people over to irvington to get to know the country better. After that was done they sended more and more Bashans to irvington and eventually took over the country. they enslaved the people of irvington because Bashan was poor so they made other people do the work.
The proper motive was to enslave irvington to put them through what the Bashans have been through.