The fundamental problem is with the [independence] tag and its excerpt. As currently written, the excerpt describes "secession"; thus questions tagged [independence] could be properly retagged [secession], while others could have the [independence] tag removed for not matching the excerpt. Doing so would result in having no questions tagged [independence].
In researching the question, I found that secession does not always affect independence and independence does not always require secession. The two subject areas are distinct though related only in international politics.
My conclusion is that the tag excerpt for secession needs to be amended and the [independence] tag needs to be replaced to distinguish between "national independence" and "local independence". Following the change, the questions need to be tagged or re-tagged, or the tag removed, to reflect the excerpts.
Based on the research and comments, the following "tag map" is proposed:
The tag excerpt for [secession] contains a reference to [local-autonomy].
While [session] and [local-autonomy] will be distinct a tag search may be done to find all occurrences of both tags.
[secession] or [local-autonomy]
The above search will list all questions tagged for both types of movements.
The excerpt for secession should follow, more closely, the Wikipedia definition.
Secession is the withdrawal of a group from a larger entity, especially a political entity, but also from any organization, union or military alliance.
For secession, I suggest:
Questions about the withdrawal of a group from a larger entity, especially a political entity. Use with a country or other tag to identify the entity. For question about "local independence" use [local-autonomy], instead.
Brexit was a secession from the European Union; but had no effect on its independence because it was always free to withdraw.
As part of the UN's Decolonization movement, the UK granted Saint Christopher-Nevis-Anguilla (Saint Christopher is more commonly called Saint Kitts) more independence as "states in free association with the United Kingdom".
The moves towards independence were not always smooth, with separatist movements/campaigns occurring in Barbuda, Nevis and Anguilla. In Anguilla, this resulted in the secession of Anguilla from Saint Kitts-Nevis-Anguilla in 1969 and its reversion to British rule as a separate colony.
Anguilla's secession from the free association colony and its reversion to British rule as a British Overseas Territory did not result in independence.
- The remaining islands seceded and gained full independence as the Federation of Saint Kitts and Nevis, though it remains part of the Commonwealth.
The excerpt for independence is more problematic. In international politics, there is no significant distinction between secession and independence — secession is the act and independence is the result. However, with internal politics, independence has "levels" or "degrees". Again, the Wikipedia definition is useful.
Independence is a condition of a person, nation, country, or state in which its residents and population, or some portion thereof, exercise self-government, and usually sovereignty, over its territory.
Note, in particular, that "or some portion thereof" denies secession as a necessary action for independence.
Add a [local-autonomy] tag.
For local-autonomy, I suggest:
Questions about actions or movements that promote a greater degree of self-government within a country or political subdivision, or whether an area is independent. Use with a country tag. When the question is about full independence from a country, use the [secession] tag, instead.
For example, the Scottish independence movement gets the [secession] tag, while the Scottish devolution movement gets the [local-autonomy] tag.
Any movement calling for the creation of an autonomous area or redistribution of power to political subdivisions gets the [local-autonomy] tag.