Late Thursday and early Friday, five police officers were killed and seven were wounded, some of them seriously, in Dallas. That is the highest number of police killed at a single time since September 11, 2001.

The shootings happened as groups such as Black Lives Matter held a peaceful demonstration against police violence in Dallas. In a two-day period, two young black men were killed by the police; protests against police violence and racism were staged throughout the country on Thursday.

Dallas is one of the cities held up as a model with regard to police work and police violence. In few cities do black officers and white officers work together so closely.

'Great (white) again'

Speculations about why the shootings happened have ranged from plausible assumptions to weird conspiracy theories. The only thing that is clear is that these events have hit a country that is about to face a difficult test: a country that in November must choose between two presidential candidates who could not be more different. And, for months now, the Republican candidate, Donald Trump, has infused his campaigning with racism.

Trump and his supporters have reactivated old racist notions in a clear "us against them" campaign. When he promises to "make America great again," he means restoring white dominance. His response to the challenges of globalization is, at its heart, based on the idea of a master race.

That is the emotional backdrop against which the murder of the five policemen has occurred. And no one can say whether the United States will be capable of dealing calmly with this situation. No one can say whether the pictures from Thursday's attack on the police and the previous days' police killings of black men might not be documents of a historic turning point.

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