The second world war had a civilising influence on Buford Posey, a white man raised in the Deep South during the Depression. "When I was coming up in Mississippi I never knew it was against the law to kill a black man," he says. "I learned that when I went in the army. I was 17 years old. When they told me I thought they were joking."

Some 70 years later it's clear not everybody got that memo. Three weeks ago in Sanford, Florida, a neighbourhood watch captain, George Zimmerman, shot dead an unarmed black teen, Trayvon Martin, as he walked home from the store. Zimmerman, who is Latino, called the emergency services because he thought Martin, 17, looked "suspicious" and then, against the advice of the dispatcher, followed him. The two men fought. Martin died. Zimmerman emerged, bleeding from the nose and the back of his head, claiming he shot Martin in self-defence because he was in fear of his life.

Zimmerman was neither charged nor arrested. Under Florida's "stand your ground" statute, deadly force is permitted if the person "reasonably believes" it is necessary to protect their own life, the life of another or to prevent a forcible felony. Zimmerman weighs 250lbs and had a 9mm handgun; Martin weighed 140lbs and had a packet of Skittles and a can of iced tea. Being a young black male, it seems, is reason enough.

One can only speculate as to Zimmerman's intentions. Efforts to create a crude morality play around this shooting in which Martin is sanctified and Zimmerman is pathologised miss the point. Zimmerman's assumptions on seeing Martin may have been reprehensible but they were not illogical. Black men in America are more likely to be stopped, searched, arrested, convicted and executed than any other group. With almost one in 10 black men behind bars there are more of them in prison, on probation or on parole today than were enslaved in 1850. To assume that when you see a black man you see a criminal is rooted in the fact that black men have been systematically criminalised. That excuses nothing but explains a great deal.

Add to this lax gun laws, entrenched segregation, deep economic inequalities and a statute that endorses vigilantism, and a murder of this kind is inevitable. Indeed what makes Martin's case noteworthy is not that it happened but that it has sparked such widespread indignation beyond his immediate community. It is not at all uncommon for young black men to leave the world in a shower of bullets followed by deafening silence.

Eight kids under the age of 19 are killed by guns in America every day. While researching the stories of those who fell one November day in 2006 I ran across the story of Brandon Moore. Brandon was 16 when he was shot in the back in the middle of the afternoon by an off-duty cop moonlighting as a security guard in Detroit. The guard had previously shot a man dead during a neighbourhood fracas, shot his wife (though not fatally) in a domestic dispute and had been involved in a fatal hit-and-run car accident while under the influence of alcohol. Brandon's death was dismissed in the city's two main newspapers in less than 200 words. They never even mentioned his name. Brandon's death was ruled to be justifiable homicide. A year later the guard was still in the police force.

"We're deemed not reportable," said Clementina Chery, who runs the Boston-based Louis D Brown Peace Institute, which assists families in the immediate aftermath of shootings and works in schools to educate people about gun violence. "Black children are dispensable. Violence is expected to happen in these communities."

Thanks to the escalating outrage at Martin's death an investigation has now been launched by the US department of justice, and the state attorney's office will be sending it to a grand jury. It took three weeks, outrage and the mobilisation of thousands of people to make that happen. Apparently the facts alone did not warrant further inquiry.

The question now is whether Martin's case can gain the attention at the highest levels of the American polity. In 2009 when a well known African American Harvard professor, Henry Louis Gates Jr, was arrested while trying to get into his own home, Obama made his views known in a clumsy intervention that ended in him staging a "beer summit" with Gates and the arresting officer.

Given how rarely Obama refers to issues of race and how much there is to refer to, it was strange that he would spend his considerable moral capital in this area to defend a tenured Harvard professor whom he knew, and who was detained for a few hours.

We'll never know if Martin could have become a Harvard professor. But it would be nice to think that his short life and brutal death would receive the same kind of presidential attention.

Twitter: @garyyounge

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