Here’s something to ponder about Henry David Thoreau, whose bicentennial birthday is July 12: Was the writer on the political right or the political left?

Thoreau certainly has some progressive credentials. A principled and active abolitionist, he participated in the Underground Railroad and defended John Brown’s raid on Harpers Ferry. He spent a night in jail for refusing to pay taxes to support the Mexican-American War, which he regarded as an imperialist adventure and an attempt to extend slavery. His work, in revival half...