The clearest example I can think of is the Western Front majorly fails (say the weather is impossibly bad for a 1944 invasion of the Continent) and you end up with West and East France and a unified Soviet Germany.
Monaco-Nice gets annexed into a Mediterranean Kaliningrad. Monaco's scenery becomes the Mediterranean resort of choice for top Soviet/Warsaw Pact officials.
In addition, its location allows for extensive signals intelligence from Western and Southern Europe, plus North Africa.
Red France of course is also a good candidate.