Earlier this month, Guenaël Brohan walked into his local polling place and did something he's never done before. Slouched against his tractor, he rattles off his political views, which are so uncontroversial they don't bear mentioning. "But I voted for Jordan Bardella, and for the Rassemblement National," he says. "I wanted to send a message to our current government: it has to change. We are in a country that is going to sink." Guenaël is not alone. Indeed, his ballot was part of a far-right tsunami that has turned French politics on its head. The next two weekends will determine...