The 18th SS Volunteer Panzer Grenadier Division Horst Wessel was formed in 1944 around a cadre from the 1 SS Infantry Brigade

1 SS Infantry Brigade

The 1 SS Infantry Brigade was a unit of the German Waffen SS formed from former concentration camp guards for service in the Soviet Union behind the main front line during the Second World War. They conducted anti-partisan operations in the rear of the advancing German army and were involved in...

 and included mainly Hungarian ethnic Germans or Volksdeutsche

Volksdeutsche

Volksdeutsche - "German in terms of people/folk" -, defined ethnically, is a historical term from the 20th century. The words volk and volkische conveyed in Nazi thinking the meanings of "folk" and "race" while adding the sense of superior civilization and blood...

 (Volksdeutsche is a historical term which arose in the early 20th century to apply to Germans living outside of the German Empire).
It was used for anti-partisan

Partisan (military)

A partisan is a member of an irregular military force formed to oppose control of an area by a foreign power or by an army of occupation by some kind of insurgent activity...

 duties until it was sent to the Eastern front

Eastern Front (World War II)

The Eastern Front of World War II was a theatre of World War II between the European Axis powers and co-belligerent Finland against the Soviet Union, Poland, and some other Allies which encompassed Northern, Southern and Eastern Europe from 22 June 1941 to 9 May 1945...

, with the exception of one regiment that fought the Slovak National Uprising

Slovak National Uprising

The Slovak National Uprising or 1944 Uprising was an armed insurrection organized by the Slovak resistance movement during World War II. It was launched on August 29 1944 from Banská Bystrica in an attempt to overthrow the collaborationist Slovak State of Jozef Tiso...

 in August 1944.