The year was 1937. Supreme Court Justice Owen Roberts had to decide which was more important: his intellectual integrity, or the integrity of the court as an institution. President Franklin D. Roosevelt’s court-packing plan—which FDR billed as a measure to shore up the country’s “overworked” courts—had precipitated a showdown with the judiciary. No one doubted the plan’s true purpose was to strong-arm the justices into ruling in the president’s favor on his signature New Deal program. If the court as constituted didn’t start approving the president’s agenda voluntarily, he was going to add enough seats filled by handpicked appointees to force it into compliance. The question was whether the justices would yield to the threat.

For Justice Roberts’ more conservative colleagues—ominously dubbed “the Four Horsemen” by political opponents—the answer was a resounding “no.” Such a blatant assault on judicial independence was too much for them, the long-term viability of the court be damned. They’d sooner see the judicial branch implode than give an inch in the face of what they saw as executive tyranny. On the other side, it was clear that the court’s liberal minority—the so-called “Three Musketeers”—would continue supporting FDR’s New Deal regardless of what happened in the court-packing fight. But Roberts, a moderate former prosecutor appointed by Calvin Coolidge, was in neither camp. So the role of saving the court fell to him and, to a lesser extent, Chief Justice Charles Evans Hughes.

In the end, Roberts came through. Despite the blow his reputation would take, he began to side with the administration in virtually all the cases that came before the court, often reversing votes he had cast just a few terms before. The message was clear: The court would no longer get in the president’s way. And it worked. Despite the usual impulse of the Democratic majority in Congress to rubber-stamp anything Roosevelt advanced, Congress took heed of the court’s change of heart and killed the court-packing bill. The crisis was over.

But why did it arise in the first place? ...