THIS much can be said for Bill de Blasio’s inauguration, which featured a concentration of left-wing agitprop unseen since the last time Pete Seeger occupied a stage alone: If the waning years of Barack Obama’s presidency are going to be defined by a liberal crusade against income inequality, there’s no more fitting place to kick it off than New York City.

It’s fitting because a glance at New York’s ever-richer 1 percent, its priced-out middle class and its majestic skyscrapers soaring above pockets of squalor makes it easy enough to understand left-wing populism’s appeal.

But it’s fitting, as well, because New York also illustrates the tensions that make the war on inequality hard to wage, and suggests reasons to question whether it’s actually worth fighting in the first place.

Those tensions start with the fact that despite a run of non-Democratic mayors, the five boroughs have hardly been a laboratory for Social Darwinists in the last two decades. Instead, de Blasio’s “tale of two cities,” one ever-richer and one struggling to keep up, has been unspooling in a liberal metropolis in a liberal state surrounded by a mostly liberal region, where many obvious anti-inequality policy levers are already being pulled.