ANN CABRERA (HOST): Douglas, what do you make of the president tweeting about internal politics, essentially, while he is on the world stage, and seeing now this back and forth, what seems to be a little juvenile, no? 

DOUGLAS BRINKLEY: Extremely juvenile. You know, I did my doctorate at Georgetown in US diplomatic history, and the first thing you learn when you study diplomacy is an American president or senator, congressperson doesn't diss their own country when they go abroad. That’s what Donald Trump has done on this trip. And this whole visit has been embarrassing. I mean, think about president of the United States coming to Europe -- Great Britain doesn't want Trump there. France doesn't want him. Only Poland took him. He now is in Germany, where he -- the Germans, only about eight percent of the German people like Donald Trump. You have massive protests, a lot of it aimed not just at the G20, but at Donald Trump himself. Our first lady is holed up in a hotel, and she's not able to go and meet the other wives of the G20. And meanwhile, you know Putin is going to be getting benefits out of this meeting with Trump. And here we have the president of the United States sending crazy tweets about [John] Podesta. It is not a good scenario. We are seeing the shrinking of American prestige in the world before our very eyes. 

Previously:

On foreign trip, media figures praise Trump as “presidential” for not acting like a child

At a press conference in Poland, Trump complains that CNN and NBC are not nice enough to him

CNN pro-Trump commentator tells Trump to not bring up Russian election tampering with Putin

Fox talked about what Trump must discuss with Putin. You can guess what they didn't mention.