A glass of wine with friends can brighten the grey days of January. But in this bleak month—and others—fewer people are indulging. It’s not simply that they are passing on wine; they seem to be passing on friends.

Bleak, too, is the mood in America, where the politics feel raw and volatile. The country’s simmering tensions are playing out in places such as Minneapolis. Donald Trump shows little interest in lowering the temperature. As America prepares to mark its 250th birthday, history offers lessons for a country under strain.