We’ve all seen footage of those carefully timed explosions that bring down antiquated buildings and clear the ground for new ones. A horn sounds, the detonations begin, the building shakes, suspended for a frozen moment, and then collapses in a vast cloud of dust. That, metaphorically, is what is happening to the Obama administration’s legacy. The presidential edifice has come down in two episodes, and the dust has not yet settled from either one. The first wrecked President Obama’s most consequential policies: the Affordable Care Act and the Iran nuclear deal. The second, happening now, is crushing its reputation for...