As has been customary for decades, millions of Americans celebrated the legacy of the Rev. Dr. Martin Luther King Jr. last week. But his legacy goes well beyond a single day. 40 years ago, after one of the longest legislative battles in its modern history, the U.S. established Martin Luther King Jr. Day in recognition not only of Dr. King’s leadership but also of the Civil Rights Movement’s moral force. This year marked the 40th anniversary of the national holiday. Despite the increasingly hostile racial climate and backlash the Trump administration is engineering, America is considerably more racially integrated than...