“Everything in life is luck.” The words are attributed to Donald Trump and they continue to make regular appearances in the press notwithstanding that the president-elect long ago declared it fake news. “I never made this ridiculous statement,” he tweeted in 2014, in language that sounds much closer to the spirit of @realDonaldTrump.

Even so, it takes nothing away from Mr. Trump’s stunning Nov. 8 victory to note that he enters the Oval Office a very lucky president. For notwithstanding the formidable challenges ahead—a dangerous...