President Trump’s long rants and seemingly erratic behavior last week — which some doctors believe might have been fueled by his use of dexamethasone, a steroid, to treat Covid-19 — renewed a long-simmering debate among national security experts about whether it is time to retire one of the early inventions of the Cold War: the unchecked authority of the president to launch nuclear weapons. Mr. Trump has publicly threatened the use of those weapons only once in his presidency, during his first collision with North Korea in 2017. But it was his decision not to invoke the 25th Amendment and...