The opinion that this phrase refers to is Justice Felix Frankfurter’s majority opinion in the Supreme Court case of Minersville School District v. Gobitis. Some of the law clerks for the justices in that year are said to have given the opinion that name because they felt that it was influenced by the fact that France had been defeated in the early stages of World War II as Frankfurter was writing the opinion in 1940.
In this case, the Supreme Court held that school children could be forced to participate in a daily salute of the American flag. The Gobitis family did not want their children to participate in the flag salute because they were Jehovah’s Witnesses and it was against their religious beliefs to venerate the flag. Frankfurter argued that people could not simply claim a blanket religious exemption from anything they did not want to do. He argued that, in this case, the government’s need to instill patriotism in students overrode the students’ rights to freedom of religion.
The people who gave this opinion its nickname did so because they felt Frankfurter had been frightened by the Fall of France. They felt that he was more in favor of forced patriotism than he should have been because he felt that the US might need to be united and patriotic to defend itself in case of war.