Terraform down

I share one large island and a Treaty Of Friendship with Diedre. She has a less friendly interpretation of that treaty than I, so I built a pressurized base on the narrow isthmus connecting our two factions' areas. I also built two skimship terraformers. I proceeded to "terraform down" the squares around my base that are clearly in my realm. However, I get a message that this will break the treaty, even though none of the affected squares have anything to do with her territory. What am I doing wrong?

You can get a cascading effect for a topo change, because all adjacent squares have to be no more that 1 bracket higher or lower.

Thanks! I even angled it so that all surrounding squares were mine. No change. I suppose she thought that all ocean squares were hers too.

Terraform down farther back.

I tried this too. The whole point was to sink the isthmus between us. Five tiles from the border and it was still a no-go.

Declare war and do it then. Sink her city. You'll need to defend your terraformer tho.