The idea of the scapegoat comes from the Bible. As part of the ceremonies for the Day of Atonement, a day when the Jews were supposed to atone for all of the sins of the past year, a goat was sacrificed as a symbolic way of dying for the sins of everyone. This is described in Leviticus 16. The goat carried the sins of the people and thus died for the sins of the people. This was a precursor to "the lamb of God who takes away the sins of the world," which would be the Messiah, to come later in the future. Christians believe that Jesus is the Messiah, the ultimate scapegoat for humanity.
This term, therefore, has come down to mean someone that takes on the guilt of others, or a group of people that takes on the guilt of others or, more likely, is BLAMED for the sins of others. In Obasan, the scapegoat is the Japanese-American people, who were forced to live in internment camps during World War II after the Japanese bombed Pearl Harbor on December 7, 1941. There was unfounded fear in the United States then that people of Japanese descent were conspiring with the enemy, or COULD be conspiring with the enemy, so the Japanese were rounded up, their lands were taken, their homes, their belongings, etc., and they were put into these internment camps. They became the scapegoats for the Japanese Empire, with whom the U.S. was at war. These Japanese-Americans were innocent and were not spies or terrorists.Most of them had been born in the United States and had never even been to Japan.