The Walloon Legion (French: "Légion Wallonie" ) was a collaborationist volunteer unit recruited from Belgium's French-speaking population in Wallonia and Brussels during the German occupation of World War II.
 The Walloon Legion served in the Wehrmacht, later in the Waffen-SS, on the Eastern Front on both front line and reserve duties.
The Croatian Air Force Legion (Croatian: "Hrvatska Zrakoplovna Legija" ), or HZL, also known as the Croatian Legion, was a foreign volunteer unit of the Luftwaffe raised from volunteers drawn from the Independent State of Croatia which fought on the Eastern Front between 1941–1943 in the Second World War.
 It was then absorbed by the Air Force of the Independent State of Croatia and its surviving members fought back on Croatian soil.
 The legion had approximately 360 men.
The Eastern Front of World War II was a theatre of conflict between the European Axis powers and co-belligerent Finland against the Soviet Union, Poland and other Allies, which encompassed Northern, Southern and Central and Eastern Europe from 22 June 1941 to 9 May 1945.
 It has been known as the Great Patriotic War (Russian: Великая Отечественная Война , "Velikaya Otechestvennaya Voyna") in the former Soviet Union, while in Germany it was called the Eastern Front (German: "die Ostfront" ), the Eastern Campaign ("der Ostfeldzug"), the Russian Campaign ("der Rußlandfeldzug"), or the German-Soviet War by outside parties.
Prit Buttar is a British general practitioner and writer.
 He has written five books: "Battleground Prussia: The Assault on Germany's Eastern Front 1944-45" (2010), "Between Giants: The Battle of the Baltics in World War II" (2013), "Collision of Empires: The War on the Eastern Front in 1914" (2014), "Germany Ascendant: The Eastern Front 1915" (2015), and "Russia's Last Gasp: The Eastern Front 1916-17" (2016).
 Dr. Buttar was Senior Partner at Abingdon Surgery until he moved to Scotland in late 2017.
The Indian Legion (German: "Indische Legion" ), officially the Free India Legion (German: "Legion Freies Indien" ) or Infantry Regiment 950 (Indian) (German: "Infanterie-Regiment 950 (indisches), I.R. 950" ) and later the Indian Volunteer Legion of the Waffen-SS (German: "Indische Freiwilligen Legion der Waffen-SS" ), was a military unit raised during the Second World War in Nazi Germany.
 Intended to serve as a liberation force for British-ruled India, it was made up of Indian prisoners of war and expatriates in Europe.
 Because of its origins in the Indian independence movement, it was known also as the "Tiger Legion", and the "Azad Hind Fauj".
 Initially raised as part of the German Army, it was part of the "Waffen-SS" from August 1944.
 Indian independence leader Subhas Chandra Bose initiated the legion's formation, as part of his efforts to win India's independence by waging war against Britain, when he came to Berlin in 1941 seeking German aid.
 The initial recruits in 1941 were volunteers from the Indian students resident in Germany at the time, and a handful of the Indian prisoners of war who had been captured during the North Africa Campaign.
 It would later draw a larger number of Indian prisoners of war as volunteers.
The 1st Devonshire Engineer Volunteer Corps, later the Devonshire Fortress Royal Engineers, was a volunteer unit of Britain's Royal Engineers whose history dated back to 1862.
 The unit helped to defend the vital naval base of Plymouth, and supplied detachments for service in the field in both World Wars.
 During the North African campaign in World War II, the unit's sappers distinguished themselves in bridging the Nile and clearing minefields during and after El Alamein.
 Their successors served on the postwar Territorial Army until 1969.
The Aberdeen Fortress Royal Engineers was a Scottish volunteer unit of the British Army formed in 1908.
 Its main role was defence of the Scottish coast, but it served on the Western Front during World War I.
 In the 1930s it was converted into an air defence unit, in which role it served in World War II.
The Caucasian Muslim Legion (German: "Kaukasische Mohammedaner- Legion/Kaukasische Moslem-Legion") was a volunteer unit of the German Army.
 The unit was composed of Azerbaijanis, Dagestans, Chechens, Ingushes, and Lezgins.
The Cornwall Fortress Royal Engineers, was a volunteer unit of Britain's Royal Engineers formed in 1908.
 It helped to defend the coastal towns of Cornwall and sent engineer units to work on the Western Front.
 Converted to an air defence role before World War I it served as an searchlight unit during the Battle of Britain, the Blitz, then as a light anti-aircraft gun unit it served in the most heavily-attacked part of the South Coast of England throughout 1942–44, including the V-1 flying bomb campaign (Operation Diver).
The 11th Regiment Illinois Volunteer Infantry was an infantry regiment that served in the Union Army during the American Civil War.
 It was originally formed as a three-month volunteer unit at the beginning of the war, and then reorganized as a three-year unit, in which role it served until the end of the war.

Question: Walloon Legion, a collaborationist volunteer unit, that served on the Eastern Front, in which war?
Answer: World War II