LVIV, Ukraine—The Pryimenko family’s bags had been packed for five days on March 7, when Russia and Ukraine negotiated a green corridor for civilians to flee Sumy, which would open the next morning. Vitaliy and Yekaterina Pryimenko had been waiting for a day to flee with their three sons, aged Kiril, 5, Yegor, 10, and Artem, 15. They stayed put, having heard stories of civilian vehicles being shot at as they tried to escape Sumy, a city in east Ukraine which is less than 25 miles from the Russian border. Vitaliy’s mother, Valentina Pryimenko, refused to go. On the evening of March 7, she reluctantly agreed to flee after a phone call with her daughter, Anna Pryimenko, who had already fled to western Ukraine. Roughly two hours later, a bomb blast killed the entire family, reducing their two-story house to rubble. Anna Pryimenko’s phone rang at around midnight. Her brother’s …