Vice President Kamala Harris bungled a history lesson about the Juneteenth holiday Monday, telling kids that black people were enslaved in America for 400 years — overstating the actual time period by more than 150 years. “I think that we all know today is a day to celebrate the principle of freedom,” Harris told a group of about two dozen elementary school-aged children at the National Museum of African American History & Culture in Washington. “And think about it in terms of the context of history, knowing that black people in America were not free for 400 years of slavery.”...