Last week, Donald Trump clutched a glowing orb with two autocrats on a glorified movie set in Riyadh, shoved the prime minister of Montenegro out of his way at a photo op in Brussels and rode in a golf cart instead of walking with other world leaders at the G7 summit in Taormina. Yet as mortifying as those moments were, it was what Trump didn't do or say during his first voyage abroad as president that could prove infinitely more destructive, not only to the United States' standing in the international community, but to the planet itself.

In his Monday column for the New York Times, Paul Krugman looks back on a diplomatic excursion that may have spelled mankind's demise. Trump made headlines Thursday by declining to affirm Article 5 of the North Atlantic Treaty, which says an attack on one NATO member is an attack on all. U.S. National Security Adviser H.R. McMaster subsequently said that "of course" the president supports the mutual defense doctrine, but the damage already appears to be done. German Chancellor Angela Merkel declared Sunday that the European Union can no longer depend on the United States and "must take its fate into its own hands," possibly by developing a nuclear weapons program.

"In effect, he repudiated the central plank of America’s most important alliance," writes Krugman. "Why, it was almost as if he’s more interested in appeasing Vladimir Putin than he is in defending democracy."

If upending 70 years of relative peace and prosperity across the west weren't calamitous enough, the president followed his NATO appearance by refusing to endorse the Paris climate agreement, a pact negotiated by 195 countries in accordance with the United Nations to reduce dangerous greenhouse gas emissions. In Krugman's estimation, the president's reasoning is as shortsighted as it is harebrained. A failure to comply with the treaty's regulations won't boost American industry, as the president and Republican leaders have claimed. And within years or even months, it will become abundantly clear that the coal jobs Trump has promised to restore are never coming back.

"Maybe Trump won’t really pull the plug on Paris; or maybe he’ll be gone from the scene before the damage is irreversible," Krugman notes ruefully. "But there’s a real possibility that last week was a pivotal moment in human history, the moment when an irresponsible leader sent the whole world careening off to hell in a golf cart."

Read Paul Krugman's column at the New York Times.