I performed the merge while indicating they were synonyms. I think that was the appropriate response. Thanks for raising this.
- cultural-history should be a synonym of culture
- political-history should be a synonym of politics
- social-history should be a synonym of society
And many more. If you are able, please check out the tag-synonyms page and vote on proposed synonyms. Although honestly the limitations are kind of a pointless pain.
(My preference is for the shorter tags to be the main one due to the precedent of economy being the main tag for economic-history. Plus naming tags topic-history seems quite redundant when the site is explicitly about history. But this is quite a minor point.)
However, there are a decent amount of questions under world-war-2. So should it be a synonym, or just merged?
At the moment I'm feeling partial to making it a synonym (or rather doing both), as it would help taggers with searching.