What does "fatal: refusing to merge unrelated histories" mean?

The error message fatal: refusing to merge unrelated histories in Git means that you are attempting to merge two branches that do not share a common commit history. This typically occurs when:

1. Two separate Git repositories were created and one is being treated as a remote branch of the other.

2. A local repository was initialized independently of an existing remote repository and you try to merge branches from that remote without a common base commit.

3. A branch was created without branching off an existing branch and is now being merged to another one.

Git's default behavior is to only allow merging of branches when there is a common ancestor commit. When Git cannot identify a common ancestor, it refuses to merge to prevent unintended loss or corruption of data.

To resolve the error, you have a few options:

1. Use --allow-unrelated-histories flag: You can force Git to perform the merge despite the lack of common history by using the --allow-unrelated-histories flag. This option should be used with caution. It treats the merge as if you were merging two unrelated projects, and you might have to resolve a lot of conflicts. For example, if you are trying to merge the branch feature-branch into main you would use:

git merge --allow-unrelated-histories feature-branch

2. Rebase Instead of Merge: Another option is to rebase one branch onto another. However, you should only rebase if you know what you are doing. Rebasing is recommended only when you are working on a feature branch and not a main branch since it alters the history of commits. You might find it helpful to rebase your feature branch against the main branch, for example:

git rebase main

3. Correcting Initialization: If the issue occurs because you've mistakenly treated two separate repositories, you might need to reconsider your repository structure and the relationship between them.

In summary, the "fatal: refusing to merge unrelated histories" message in Git is a security feature. It means that Git has recognized that the branches you are trying to merge do not share a common commit history. Using the --allow-unrelated-histories flag, or using rebase can help you proceed, but you should proceed carefully and be aware of the implications. Usually it is better to understand and correct your git structure rather than just force a merge by using --allow-unrelated-histories

Learn more about this