‘I saw the tents of Cushan in affliction,
The curtains of the land of Midian trembled.'
Midian combined with Moab in seeking to prevent the passage of Israel into the promised land (Numbers 22:4; Numbers 22:7). They were a feared nomadic people, and vexed Israel with their wiles (Numbers 25:18), while their women led Israel astray (Numbers 25:6). But they had cause to tremble for YHWH commanded Israel to smite them (Numbers 25:17; Numbers 31:2-9) because of their activities against them, and in order to prevent any further mischief. The remainder (for they were widespread) feared as Israel entered the land.
Cushan must in some way be related to Midian, possibly a large Midianite tribe, large enough to be seen as representative. The trembling of the curtains may represent their women peering out in fear.