WARSAW, Poland — President Barack Obama on Friday honored the memories of those slain in the Warsaw Ghetto uprising against Nazis, telling one elderly man that the memorial was a “reminder of the nightmare” of the Holocaust in which 6 million Jews were killed.The president also helped placed a wreath at the Tomb of the Unknown Soldier, dedicated to all unidentified soldiers who have given their lives to Poland in past wars. By paying homage to Poles who fell in World War II at two symbolically potent sites, Obama’s gestures were sure to carry great weight in a country whose identity is still profoundly shaped by the death and destruction inflicted on it by Nazi Germany. In the final phase of his European trip, the president greeted Holocaust survivors and leaders of Poland’s Jewish community at the Monument to the Ghetto Heroes. He smiled, shook hands and hugged those gathered under a light rain, including some who shared memories of having met Obama at earlier times.“What a wonderful visit. I’ll have to bring my daughters,” Obama said as he exited the memorial. The monument in the former Jewish ghetto commemorates the tens of thousands of Jews killed in a 1943 uprising against the Nazis during Germany’s brutal occupation of Poland during World War II. Most of the insurgents in that uprising were killed, but the event bears great importance in Jewish history as an example of Jews bravely taking up arms to defend themselves against the Nazis. It’s also a key memorial in a country that before the Holocaust was home to Europe’s largest Jewish community.