There is one presidential duty that each incumbent must eventually bear, but whose timing no one can foresee. Sooner or later in each presidency, something sudden and terrible will occur—an attack, a natural calamity, something as inevitable as a mass shooting or as unusual as a space-program disaster.
In response to these sorrows and emergencies, previous presidents have understood a specific, very important part of their job. At least for a moment, they must shift from head of government to head of state—leader of us all—and reflect the shared sentiments of supporters and critics alike.
In this role (and using examples from only the past few decades), a president can express a broad sense of national grief—as, for instance, Barack Obama did after the 2015 Mother Emanuel gun massacre in South Carolina, in which a 21-year-old white supremacist murdered nine black parishioners at worship. In this role a president can express a sense of national resolve, as Ronald Reagan did after the on-live-TV explosion of the space shuttle Challenger in 1986. Or as Bill Clinton did after an antigovernment terrorist killed 168 people by bombing a federal office building in Oklahoma City in 1995. And in this role a president can reassert enduring national principles, as George W. Bush did in the best speech of his presidency, an address to a joint meeting of Congress nine days after the 9/11 attacks in 2001.