Trump has picked fights over the flag before. But this time was different.

When President Donald Trump decided, out of the blue, to attack the National Football League over its players’ protests during the national anthem, the resulting controversy followed a well-worn formula. What was different, however, was the enormous backlash that his comments created — far larger than any of those previous incidents combined.

President Trump attacked an enormously popular sport whose fans prefer it to be a politics-free arena, while once again touching on the raw nerve of race. In so doing, the president proved anew that divisive provocations can mean something completely different when they come not from a private citizen, but the man whose very job description is to lead the country. “Most presidents believe that a big part of their job is to keep the country together,” said Michael Beschloss, a presidential historian, who noted that even Richard M. Nixon spoke of bringing the nation together during his 1969 inauguration. “There is very little sign that Donald Trump has much of an idea that unifying this country has much to do with being president. He just hasn’t shown it.”