Git: How to merge two histories with no common base

If you have already encountered this kind of error message with Git:

fatal: refusing to merge unrelated histories

This probably means that something wasn't done correctly… But that's okay!

The problem

This error often happens (but not exclusively) at the beginning of a project.

For example, you retrieve an empty project from GitHub/GitLab/… and you start working directly on a dev branch.

At the same time, you add a file (README.md for example) on the main branch.

But when it comes time to do a git merge, Git will refuse to do anything, because the base of the two histories is completely different.

Fortunately, the solution is very simple

The solution

To force Git to merge the two histories, you just need to rerun your git merge command with the following option: --allow-unrelated-histories

In the case where certain files exist in both branches with different contents, then you will need to resolve the conflict.

But once that's done, your merge will be perfectly functional!

Good to know

Previously, git merge agreed to merge two histories even when they were different, but this feature has changed since version 2.9

See the changelog

The git pull command, on the other hand, automatically passes the --allow-unrelated-histories option by default, which avoids this kind of problem, especially at the beginning of the project.