By Philip Pullella VATICAN CITY (Reuters) – A sombre Pope Francis on Sunday issued his toughest condemnation yet of the invasion of Ukraine, saying the “unacceptable armed aggression” and “massacre” must stop. The pope has not used the word “Russia” in his condemnations of the war since President Vladimir Putin ordered the invasion on Feb. 24. But the pontiff’s choice of words appear increasingly aimed at rejecting Moscow’s justifications for the invasion. “Faced with the barbarity of killing of children, of innocents and unarmed civilians, no strategic reasons can hold up,” he told 25,000 peo…