Dallas shooting: Who was Micah Xavier Johnson? 
The murder of five officers by a sniper in the Texan city of Dallas last week was the convergence of three of America's great unsolved problems.
The immediate chain of events began on Tuesday, the day after Americans had celebrated their proudest patriotic moment, Independence Day.
A black man was shot and killed by a white police officer on the footpath in front of the convenience store where he plied his trade selling music CDs in the city of Baton Rouge in the southern state of Louisiana. 
Phone footage shows two policemen holding Alton Sterling, 37, father of five, to the ground when one of the officers draws his revolver and shoots him in the chest.
A day later, another black man was shot and killed by a white police officer, this time in St Paul in the northern state of Minnesota. 
Philando Castile, 32, a school cafeteria assistant, was pulled over while driving.
The encounter ended with a policeman firing bullets into Castile through the open car window as his girlfriend and young daughter looked on.
The next day, Thursday, protests convened in cities across America. Some 800 people turned out at the one in Dallas.
During the peaceful demonstration, shots suddenly rang out and the first of 12 law enforcement officers fell to the ground, shot.
The police present at the rally, frantically trying to work out where the shots were coming from, detained three of the protesters who had AR-15 semi-automatic rifles slung over their shoulders, openly and legally under Texas law.
Not because they were shooting but in case they were connected to the gunman, the Dallas police chief later said.
The shooter, using a semi-automatic rifle legally available in Texas, eventually killed five officers.
The mass murderer, a black man, told the police that he had been targeting white police officers, the police chief said.
He was later found to have been a supporter of the New Black Panther Party, a group that advocates the use of violence against white people generally and Jews in particular.
The killer, Micah Johnson, 25, was an army veteran. So were four of the five officers he murdered that night.
In the aftermath, with anger and dismay across American society, a former police chief, Charles Ramsey, a black man, told the media that the country was sitting on an explosive:
"You can call it a powder keg. You can say that we're handling nitroglycerin. But obviously when you just look at what's going on, we're at a very critical point in the history of this country."
The ingredients of America's nitro have been becoming more potent and more volatile since Martin Luther King identified them in 1968.
It's well known that King's nonviolent protest movement led to greater legal equality for America's blacks. It's less known that he regarded this as just a beginning.
It was in 1968, the last year of his life, that the Nobel laureate identified the great struggle ahead. The US was suffering systematic "racism, poverty, militarism and materialism".
It's the first three of these that present America with its particular brand of danger today. The fourth, materialism, is present too, of course, yet it is a universal feature in the developed nations of the world, yet most of them have nothing of the American explosiveness. It's the first three that seem to combine with unique potency.
In 1968, after tremendous success against racial segregation, King turned his full attention to economic inequality. He called it the "Poor People's Campaign". He wanted an "economic bill of rights".
On the night before he was assassinated, in his last speech, King said: "God has commanded us to be concerned about the slums down here, and his children who can't eat three square meals a day."
He told his audience that black Americans might be poor individually, but that they had a collective purchasing power of over $US30 billion, greater than total US exports.
He called for a boycott of Coca-Cola, a big milk company and two bakeries for mistreating black workers.
"We begin the process of building a greater economic base. And at the same time, we are putting pressure where it really hurts."
Without King to lead it, this economic campaign collapsed. It's no coincidence that today's heightened tensions follow a long stagnation for middle and low income Americans and the aggravation of grotesque inequalities.
Barack Obama two years ago described inequality as "the defining issue of our time".
After the Dallas shootings, Obama pointed squarely to entrenched racism as a contributing factor. The shootings, he said, were "symptomatic of a broader set of racial disparities that exist in our criminal justice system".
The President persisted, too, with his effort to draw attention to America's peculiar psychosis, its crazed embrace of ever-more deadly weapons. Guns were not the only cause, he said, but they were one cause. In the face of Congressional obstruction, Obama has proved largely impotent on all three issues.
Just as King feared - inequality, racism and militarisation, or at least, a country bristling with weapons of war - are the elements of the nitroglycerine of a potential American explosion.
Unfortunately, America today has no leader of the stature of King to seek a solution. Calm unity is required in a system that has is increasingly fired with angry division.
Australia cannot afford to be too smug about the tribulations of its ally. Australia today has more illegal guns in circulation than it did when John Howard launched his crackdown. He has lately urged renewed action. Australia's relatively benign levels of inequality are not fixed; inequality, in any country, is a simple matter of policy choice by government. And virulent racism is only ever a few Pauline Hansons away.
Australians can only wish America well, but we can take more active preventative measures at home.
Peter Hartcher is international editor.