In 1938 Neville Chamberlain, then PM of GB, met with Hitler in Munich. The result: Chamberlain, in an extraordinary show of appeasement and servility, gave Hitler the Sudetenland and everything else he wanted in exchange, essentially, for Hitler's word that Germany would play nice.
Many feel that Chamberlain's submission not only emboldened Hitler but, in retrospect, made WW2 inevitable.
In 2017 Trump, the president of the US, met with Vladimir Putin in Hamburg.
Trump did not challenge Putin on:
Russian interference with western elections including especially in the US, through use of social media
Russia's annexation of the Crimea
Russia's attempt to compromise US voting machines
Russia's hacking into email accounts of major American figures
Or on anything else.
Basically Trump, like Chamberlain 79 years before, met with the world's most dangerous criminal head of state and treated him not only with respect, but obsequiously gave him everything he wanted. Trump asked for nothing, got nothing, and left uttering empty platitudes about a working relationship and absurdities about cooperation toward the end of cyber security.
This is no small disaster.
After GWB Hillary Clinton, as SoS, had to travel the world to repair the reputation of the US.
The damage done by Trump may put out of reach a similar repair job by the SoS of the next responsible US administration.
The US, under Trump, may have already ceded, for at least a generation, the US's president's special role of leader of the free world.
Even worse, due to Trump's ridiculous performance on the world stage in Hamburg, we are much closer to WW3 today than before the G20, now better named the G19.