Russian intervention in the Franco-Prussian War

What if Russia broke the post-Crimean settlement and intervened in the Franco-Prussian War?

Early intervention could trap Prussia between France and Russia, while a late move would leave Russia facing a victorious Prussia after France had already been defeated.

Timing determines whether Russian intervention could help France. Early intervention would catch Prussia while its armies were still defeating France and force it to address a new eastern threat before victory was secure. Late intervention would find France already beaten, allowing Prussia to redirect against Russia. Britain is imagined watching the resulting balance-of-power crisis and deciding how to exploit it. Depending on when Russia repudiates the post-Crimean settlement, the scenario changes from a rescue of France to a separate Prusso-Russian war.