Why did Ruth sigh when she heard the song from the group of soldiers?

Ruth, Edik, and Bronia are making their way through the Russian-controlled zone of Berlin. As they wait to cross a bridge over the river Elbe, the children encounter thousands of victorious Soviet troops heading in the opposite direction. Ruth has never seen so many soldiers before.
Some of the soldiers are singing an old Cossack song, which Bronia instantly recognizes; it was taught to the children by their father. Ruth also remembers the song; it brings back happy memories of that glorious, idyllic summer spent by the Dunajec River. There, the whole family were on a raft, floating downstream between the majestic wooded peaks. Ruth sighs as she reflects on those happier times before the Nazis invaded Poland and tore her family's life apart.