Already, more than a thousand people have signed up to be arrested over two weeks beginning Aug. 20 — the biggest display of civil disobedience in the environmental movement in decades and one of the largest nonviolent direct actions since the World Trade Organization demonstrations in Seattle back before Sept. 11. (Among the first 500 to sign up, the biggest cohort was born in the Truman administration, followed closely by FDR babies and Eisenhower kids. These seniors contradict the stereotype of greedy geezers who care only about their own future.)

The issue is simple: We want the president to block construction of...