S'ils ne la connaissent que de nom, voici une vue de cette rivière en Biélorussie.

June 11, 2017, there were elections in France, and the former socialist party of François Hollande suffered a very strong defeat.
The expression "It is the Berezina" refers to the heavy defeat of Napoleon Ist against the Russian soldiers in 1812. When the French troops wanted to cross the Berezina, this Russian river was frozen, and Napoleon's troops could not cross it. 45,000 soldiers died of cold and hunger.
This is the explanation of the expression used in France to speak of a great defeat!