Water-rich Switzerland controls Western Europe’s taps — and wants it to stay that way. Its drought-ridden neighbors are getting nervous.

At the western edge of Lake Geneva, where the mighty Rhône river squeezes through a narrow dam, a blunder of French diplomacy is carved into stone for all to see.

The inscription, mounted on the walls of an old industrial building, commemorates the 1884 accord between three Swiss cantons that have regulated the water levels of this vast Alpine lake ever since. It does not mention France — even though some 40 percent of the lake is French territory.

“France, for some reason, wasn’t part of the contract,” said Jérôme Barras as he unlocked a gate below the epigraph to inspect a hydropower plant under the dam he has managed for more than a decade.

When the agreement was renewed and a new dam was built a century later, Paris still wasn’t interested.

The French government now regrets that.

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And France has suddenly realized it can’t control that tap as it battles water shortages, destructive droughts and baking heat.