When the Yale University Library mailed the envelope above it was only days before the Japanese attack on Pearl Harbor which precipitated the entry of the U.S. into World War II. The envelope was mailed on December 3, 1941 to the Library (Bibliotheque) of the Universite Libre in Brussels, Belgium. U.S. mail to Europe was already being examined by censors and the label on this envelope indicates that this was accomplished by Examiner 5141. Belgium had been invaded by Nazi Germany in 1940 and was an occupied country. According to the website of the Universite Libre de Bruxelles the university closed its doors in 1941 to avoid collaborating with the Nazis. The envelope has what philatelists refer to as an auxiliary marking - "Return to Sender Service Suspended". Postal service was suspended by many countries during World War II. The envelope was returned through New York City and has a date stamp of July 28, 1942, almost eight months after it was first mailed. Since the envelope doesn't include any contents, there is no way of knowing for what purpose it was sent. An interesting piece of postal librariana.