Close Strategic Bases?

How to close strategic base?

It says defense is too weak and gives no option to close it. Should I increase my defense to a certain level, just to close a strategic base?

I don't think once you've built one that you can close down to zero.

Yankee is correct. The last military base of any category cannot be closed.

This is the way it is to avoid the possibility of weapons/ammo ending up in limbo when there is no more base to house them.