I take it with a PoD after May 1945?
I'd say you need several smaller things, reinforcing and escalating each other. Take together perhaps:
- Some "incidents" in the French occupation zone. Nothing Earth-scattering just causing resentment. Perhaps "aided" by French politicians talking about how them Krauts got it coming. Germans talk about how the Saar suffered under the French jackboot a lot more often than France was invaded by Germany. Grumbling all around.
- Clement Attlees Labour gouvernment decides that faced with a huge New Russian Empire, Britain is no longer kept safe by seeking a disunited Europe. Instead Western Europe must be under British Leadership.
- Add some other butterflies and you could see an EEC with Britain replacing France as one of the six founding members.
- France then like Britain OTL huffs, puffs and stays out. Later when they want to join Germany like France OTL vetoes it.
- France holds on tighter to it's de-facto colonial Empire after de-jure decolonization. Drawing the ire of the German Left aka the part who'd otherwise be pushing strongest for making concessions.
- France eventually joins the EEC, but never has more power than Spain or Italy. Germany and the UK are firmly in the drivers seat.
That should be enough for fences never to really be mended.
After the Fall of the Warsaw Pact, there's no Euro, since Britain has no interest in demanding that from Germany as a price for unification and no one in Germany cares about French disapproval. The Deutschmark becomes the unofficial parallel currency in much of Eastern Europe and the Balkans. Doesn't turn Germany into a superpower, but gives it enough clout to count as "geopolitical rival" to France IMHO.
