In the mid- to late-1960s, America experienced an astonishing string of “long hot summers”: deadly riots in Watts (Los Angeles), Cleveland, Newark, Detroit and Washington, DC.

In 1992, south-central Los Angeles suffered similar outbreaks of violence after a jury dismissed a blatant case of police brutality against motorist Rodney King caught on videotape.

In 2014, violence erupted in Ferguson, Missouri, after the shooting of unarmed teenager Michael Brown. In New York, Eric Garner was choked to death while pleading with police that he could not breathe.

In 2015, Baltimore was ablaze when Freddie Gray died of a spinal cord injury in police custody a week after being chased, arrested and injured by police for no crime other than running away.

To be sure, there has been impressive growth of a prosperous black middle class since the 1960s. The US army boasts many senior black officers. Interracial marriage, a key indicator of harmony, is increasing. More African Americans are enrolling in college, including the Ivy League universities. And as protests erupted across the US, large numbers of young white Americans joined the "Black Lives Matter" campaign.

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However, the deepening troubles of the inner-city poor are spreading despair and suspicion. President Donald Trump, far from leading the healing, is fanning the flames of resentments and hatreds while the quiet voices of the thoughtful people of goodwill — the folks who want to build bridges — are in short supply.

Something more is going on here than resentment at police brutality — it is a crisis of confidence that goes beyond Trump — and it’s just not clear how Americans today can resolve their differences.

Despite Trump’s pledge of making America great again, the nation is badly off track and there is a haemorrhage of confidence in its future.

As bad as things are now, imagine what happens when the gravity of the looming economic crisis is evident. Americans from anarchists and socialists on the far left to far-right extremists have powerful incentives to get their pitchforks out and bring the system down.

In this election year, it’s a fair bet that America will become more divided, partisan and polarising. Former defence secretary and CIA director Robert Gates once remarked that the "greatest national security threat" to the US "is the two square miles that encompasses the Capitol Building and the White House".

Gates made those remarks in 2014, well before Trump arrived on the scene. If he thought congressional gridlock and infighting was a big problem then, it is bound to become an even bigger problem in 2020. The crisis playing out across Minnesota and other American cities will reaffirm these trends.

Tom Switzer is executive director of the Centre for Independent Studies and a presenter on ABC Radio National.