Judges 6:3

‘ And so it was, that when Israel had sown, the Midianites used to come up, and the Amalekites, and the children of the east. They used to come up against them.'

The land was seen as an easy target with no strong defenders to prevent their marauding. Their aim was partly booty, including cattle and wealth, but partly revenge, for they came to destroy the crops not to make use of them. Another reason for this latter, however, would be to keep the land weak. They ‘came up' because they came up from the Jordan rift valley.