We have seen looting, burning buildings, tear gas draping even the White House, smoke and flames across the country.

The rioting started after the death of a black man, George Floyd, who died when a police officer knelt on his neck for nearly nine minutes, even as on-lookers said that the man was suffocating and he himself was saying "I can't breathe".

The video of the death (for which the officer has been charged with homicide) went viral and the protests took off, turning to riot in some cases and to looting in some instances.

Initially, hundreds of people, both black and white, went to the place where the death occurred. Demonstrators chanted "I can't breathe," and "It could've been me".

Demonstrations turned violent and spread across the country.

There is a long history of violence against black people in the United States, starting with slavery and going through to lynching. The last recorded lynching was in 1981.

On recent official figures, black people are much more likely to be jailed than white people. Roughly a third of those behind bars in America are black even though black people make up only 13 per cent of the population.

According to the National Association for the Advancement of Colored People, "African Americans and whites use drugs at similar rates, but the imprisonment rate of African Americans for drug charges is almost six times that of whites."

Although the gap has narrowed in recent decades, a wide disparity remains between black people and white people in terms of the main measures of poverty - life expectancy, income, wealth and unemployment.

The "Black Lives Matter" movement started after the fatal shooting of an African American teenager, Trayvon Martin, in 2012. The white man who pulled the trigger was acquitted of murder.

There have been countless deaths of black Americans in confrontations with the police. In March, for example, Breonna Taylor was shot eight times after Louisville drugs detectives knocked down the front door. No drugs were found in her home.

Some of these situations are complex, happening in a highly charged environment, with the possibility of guns being present. Sometimes black police officers are involved.

But there is no doubt, either, that young black men are more likely to be killed on arrest than young white men.

As the Los Angeles Times put it: "About one in 1000 black men and boys in America can expect to die at the hands of police, according to a new analysis of deaths involving law enforcement officers. That makes them 2.5 times more likely than white men and boys to die during an encounter with cops."

African Americans have died at a rate of 50 per 100,000 people compared with 21 for whites, according to the non-partisan APM Research Lab.

More than 20,000 African Americans have died from COVID-19, about one person in every 2000 of the black population of the United States.

Few would argue that it does but it gives a background to a level of anger.

Rioters may have a variety of driving forces - blind anger, political activism, greed if the riot is a shield for looting.

Riots are often triggered by events but then turn in wild, unpredictable ways. Organisers of protest lost control.

There were riots after the assassination of Martin Luther King Jr (a man who shunned violence). The 1992 riots in Los Angeles started after a jury acquitted police officers who had been recorded on tape repeatedly beating Rodney King.

At a local level, governors and mayors have often tried to defuse the situation. In Atlanta, for example, Mayor Keisha Lance Bottoms made an impassioned speech condemning both the death of George Floyd but also the rioters. "What I see happening on the streets of Atlanta is not Atlanta. This is not a protest. This is not in the spirit of Martin Luther King, Jr. This is chaos."

Local law enforcement agencies have tried to contain the violence. The National Guard made up of military reservists has been deployed, all without success.

Mr Trump said he was "rightly sickened and revolted" by the death of Mr. Floyd, adding, "If a city or state refuses to take the actions necessary to defend the life and property of their residents, then I will deploy the United States military and quickly solve the problem for them."

But his opponents accuse him of fueling the situation.

He showed sympathy for armed, right-wing protesters who occupied a government building in protest at Michigan's coronavirus lock-down, calling them "very fine people".

Academic researchers found that "Donald Trump's election in November of 2016 was associated with a statistically significant surge in reported hate crimes across the United States".

In contrast, he has painted the current protests purely as violent and lawless, saying "when the looting starts, the shooting starts", echoing a 1960s police chief who championed violent police crackdowns.

Riots are complex. These ones will burn themselves out but the bitterness will remain for decades. The background of deep racism matters but also looting is crime.

Nobody can doubt, though, that they show that the United States is deeply divided and damaged.