Board Thread:Fun and Games/@comment-2A00:23C4:2ED2:C200:1C8B:A1A8:F896:CC29-20181103130857/@comment-3152371-20181103143845

If a city are just friendly (blue face) instead of ally (green face), you can give them gifts to turn them into allies.

When all remain cities are allies (no friendly or lower city left), they will merge to your civilization.

And, yeah, attack them can also work.