It seems it does not work.

not at peace in skirmish.

not at war

There is no non-agression pact.

https://www.reddit.com/r/HumankindTheGame/comments/p6x6yf/under_one_banner_not_working/

1. I select an army (even if I abort ransacking)

2. click on the highlighted "under one banner" would click on the outpost (ayeyarwady) or territory to claim it.

3. nothing happens