Louis (or Aloisio) Sonntag lived in the 19th century and was likely a member of the Papal Zouaves, the military force of the Papal States.

Prior to 1870, the Roman Catholic Church controlled and governed territory in several regions of what is now Italy, and the Pope was the head of state for these regions, collectively referred to as the Papal States. During the mid-1800s, Italian nationalist movements sparked revolutions that clashed with the Papal government, and these territories were gradually taken from Papal control. By 1870, only Rome remained under Papal control, and the Italian government officially declared war and captured the city. The Pope (Pope Pius IX) retreated to the Apostolic Palace and retained some soveriegn powers over the area known as Vatican Hill until the State of Vatican City was officially created in 1929.

The Papal Zouaves were a volunteer military force comprised of soldiers from many Catholic countries, and were charged with the defense of the Papal States during the conflicts with Italian forces. In 1870, after the taking of Rome and all other Papal States by the Italian government, the Papal Zouaves were disbanded.

Louis Sonntag was probably German or Austrian, and served the Papal State in some military capacity during the 1850s. In 1876 he visited Philadelphia.