American democracy is fragile, and unless care is taken it could follow the path of Nazi Germany in the 1930s.

Mixed in with many softer comments, that was the somewhat jaw-dropping bottom line of Barack Obama last night as, in a Q&A session before the Economic Club of Chicago, the Chicagoan who used to be president dropped a bit of red meat to a hometown crowd that likely is a lot closer to him than the man whose name never was mentioned: President Donald Trump.

Obama's comments came after a series of playful questions from moderator and Ariel Investments President Mellody Hobson—in the great Batman vs. Superman debate, for instance, we learned Obama sides with Batman—before she eventually asked him what he's learned as a world citizen of sorts.

One thing he's learned is that "things don't happen internationally if we don't put our shoulder to the wheel," Obama said, speaking of the U.S. "No other country has the experience and bandwidth and ideals. . . .If the U.S. doesn't do it, it's not going to happen."

Obama gave one specific example, but it was a solid one: Ebola. To fight the virus the U.S. did everything from build an airport tarmac in Africa to send in medical teams and ferry medicos from other countries. "We probably saved a million lives by doing that," he said.

At least indirectly, those comments could be seen as criticism of Trump, whose foreign policy focuses on an "America first" paradigm that critics say distracts from this country's unique role.

Obama moved from that to talking about a nativist mistrust and unease that has swept around the world. He argued that such things as the speed of technical change and the uneven impact of globalization have come too quickly to be absorbed in many cultures, bringing strange new things and people to areas in which "people didn't (used to) challenge your assumptions." As a result, "nothing feels solid," he said. "Sadly, there's something in us that looks for simple answers when we're agitated."

Still, the U.S. has survived tough times before and will again, he noted, particularly mentioning the days of communist fighter Joseph McCarthy and former President Richard Nixon. But one reason the country survived is because it had a free press to ask questions, Obama added. Though he has problems with the media just like Trump has had, "what I understood was the principle that the free press was vital."

The danger is "grow(ing) complacent," Obama said. "We have to tend to this garden of democracy or else things could fall apart quickly."

That's what happened in Germany in the 1930s, which despite the democracy of the Weimar Republic and centuries of high-level cultural and scientific achievements, Adolf Hitler rose to dominate. "Sixty million people died," Obama noted, "so, you've got to pay attention. And vote."

Obama said his greatest "regret and disappointment" was the failure to enact tighter controls on gun possession. Though the issue resonates in far different ways with different parts of the population, "something is broke," Obama said, his own voice breaking, as he talked about 6-year-old girls shot to death at Sandy Hook, girls not too different in age from his own daughters.

To the best of my knowledge, the session, which went well over an hour, represented the lengthiest comments the former president has made in a semi-public setting since he left office.

Admission to the event was open to club members and their guests. Hobson said a record 2,800 people attended, filling two ballrooms at the Chicago Hilton.