In his master's thesis, Krepper was very critical about the U.S. and of the church for not being outspoken against slavery during the American Civil War.
[2] In 1942, during Operation Pastorius, Krepper helped establish safehouses in New Jersey for the saboteurs of the mission.
[3] Due to a lack of evidence, Krepper was not arrested when the plot was exposed by Dasch and Ernest Peter Burger.
[1] In February 1945, Krepper was found guilty on one count of trading with the enemy, for which he faced up to 20 years in prison.
The following month, he was also found guilty of conspiracy to commit sabotage and violate wartime censorship laws, for which he faced up to two years in prison.