It is really hard to comment on the Galactic Federation.

Most people would want to say that they are the "good guys", but they are not. You might say that they understand the "plan" and are making sure the plan isn't messed with.

The reason they would stop a nuclear exchange is that, that would break the plan.

If it was just us humans involved, then they would be like, "go ahead and nuke yourselves, it is free will". But, there is more, many more races involved.