The White House keeps insisting that an unannounced discussion between President Trump and President Vladimir Putin of Russia during the recent Group of 20 meeting in Germany is nothing peculiar. In fact, history shows that it is rare for American presidents to meet other leaders, especially adversaries, one-on-one, with little evident preparation and no other American representative present. Mr. Trump’s unique affinity for Mr. Putin is, of course, an additional reason to be concerned.

The encounter occurred July 7 on the sidelines of a banquet attended by the G-20 leaders and their spouses. The president left his chair and walked over to speak with Mr. Putin. Mr. Trump said the conversation lasted 15 minutes, but other reports suggest it may have been as long as an hour. Earlier in the day, the two men held their first formal bilateral meeting, which ran about two hours and included Secretary of State Rex Tillerson, the Russian foreign minister, Sergey Lavrov, and one interpreter for each side.

Although the formal meeting was announced in advance, the later encounter was confirmed by the White House only after reports surfaced about the astonishment of some of the other guests.

It’s not unusual for leaders at such events to engage in small talk or even have short “pull-asides” in which a substantive issue is raised. If Mr. Trump were to do that with Prime Minister Theresa May of Britain, for instance, it might go unnoticed. Sean Spicer, then the White House spokesman, dismissed the exchange with Mr. Putin as purely social.