People who are not like ourselves must be treated as equals. That was Dr King's injunction, and it still resonates in the United States and the United Kingdom, three decades after he was awarded the Nobel Peace Prize. "I have a dream," he said, "that my four little children will one day live in a nation where they will not be judged by the colour of their skin but by the content of their character."
In the 15 months since I have been at Grosvenor Square, I have often been reminded that diversity challenges British society as much as America's. Walking the length of Britain, I passed thriving Thai take-aways in ancient villages. In Northern Ireland, I joined Catholics and Protestants building houses for each other through Habitat for Humanity. In London tomorrow, I will meet jobless youths from Lambeth and Brixton who are learning work skills with the Peabody Trust.
America, said Walt Whitman, is a "teeming nation of nations". From Britain, from Germany, Ireland, Italy, Russia, China, Africa and all over the globe our ancestors came. The nation's motto is e pluribus unum, out of many one.
Dr King challenged America to make good on that vision of unity and to keep our promise to the Jeffersonian ideal that "all men are created equal". Although our nations have made considerable progress since King's lifetime, we still have much to learn from each other.
In many ways, America is closer than ever before to the standard Dr King sought. For example, the unemployment rate for African-Americans is the lowest in decades. Black Americans are better educated, better housed, and better paid than 30 years ago. More than one-third of blacks live in households earning the national median income of $35,000 or more. In the 1960s, that figure was less than 10 per cent. The percentage of black women in white-collar jobs has jumped from less than 6.4 per cent in 1940 to almost 60 per cent today.
In 1940, only 7 per cent of blacks finished high school. Today more than 60 per cent of blacks graduate. The US military, where minorities comprise almost a third of the total force and 15 per cent of officers, has been a model of race relations and equal opportunity since its integration in 1948.
Amid this progress in economic and educational opportunity, however, the American dream still eludes many Americans. The challenges to our society are staggering: more than half of black children in the US, for example, live in one-parent families.
Because America's population is far more diverse now than in King's lifetime, we are grappling with new and complex questions. Should Mexican-American children be taught in Spanish? Does a Muslim sailor have the right to wear her hijab, the required head scarf, at work? (The Navy said yes.)
Although the US has five times as many ethnic minorities as Britain as a percentage of population, the UK faces many similar issues. I understand that in London there are 33 immigrant communities of more than 10,000 people. More than 200 languages are spoken here. Curry seems to be as popular as fish-and-chips, steak-and-kidney pie, or even Yorkshire pudding.
If we are to improve social and economic conditions for all our citizens, the peoples of our diverse societies must recognise our similarities, rather than dwell on our differences. If enduring peace is to be found in Northern Ireland, and if UK and US hopes for Kosovo and the Middle East are to be realised, then Martin Luther King's message must be heeded.
If the values of our children's children are to resemble, if not replicate, ours, we must ensure that our fellow citizens, who may not share our ancestry, nevertheless embrace the most cherished Anglo-American traditions and institutions. The rule of law. Parliamentary democracy. Equal justice. Open government. Freedom of religion. Individual liberty. A free press. The inherent dignity and worth of every person.
If we have sometimes failed our principles, our principles have not failed us. Martin Luther King -- from a jail in Birmingham, Alabama to the steps of the Lincoln Memorial - did not let Americans forget that.
In the East End of London, I visited a building that began as a church for French Huguenots fleeing persecution. Later it became a synagogue for Jewish refugees. Today, it's a mosque. Like that building, our institutions must endure, even as they accommodate different citizens - whatever their race, creed or colour. To keep that dream alive, we honour Martin Luther King today.
Philip Lader has been US Ambassador to Britain since December 1997. He was formerly President Clinton's Deputy Chief of Staff.