It is the great wrong that we have been trying to make right since the founding of this country: the racial divide between black and white.
It took a civil war and many lives, including that of President Lincoln, to end slavery.
It took more lives to end the segregation that came in the years after that, and this year we mark the 50th anniversary of the Selma march, one of the great turning points of the civil rights movement.
But that was far from the end of it.
There was Watts in 1965; the riots across the country after the assassination of Martin Luther King in 1968; Los Angeles in '92.
Then, Ferguson last year ... and now Baltimore.
The laws were changed, but in too many places not the attitudes.
The trials of O.J. Simpson forced us to recognize that whites and blacks can see the same things and come to different conclusions. Polls showed many whites thought Simpson guilty; many African-Americans had an opposite view.
I will always believe that people of good will on both sides want us to be one country, one great community where the whole is greater than the parts.
But the lesson of Baltimore is we are not there yet. This goes beyond a misunderstanding between police and African-Americans; this is about all of us.
The divide remains, and there is still much to do for people on both sides.