Question: In which country did Anton Ludwig August von Mackensen lead an Army Group from 1916-1918?

Context paragraphs:
Battle of Dunajetz: The Battles of Dunajetz occurred in May 1915 as part of the Gorlice–Tarnów Offensive during World War I.  They were fought between the Russians (who were camped out along the line of the Dunajec river, the right tributary of the Vistula) and a force led by German General August von Mackensen to push the Russians out of Galicia and Vistula land (what is now Poland).

August von Mackensen: Anton Ludwig August von Mackensen (6 December 1849 – 8 November 1945), born August Mackensen, was a German field marshal.  He commanded with extreme success during the First World War and became one of the German Empire's most prominent and competent military leaders.  After the Armistice, Mackensen was interned for a year.  He retired from the army in 1920 and was made a Prussian state councillor in 1933 by Hermann Göring.  During the Nazi era, Mackensen remained a committed monarchist and sometimes appeared at official functions in his First World War uniform.  He was suspected of disloyalty to the Third Reich, although nothing was proven against him.

Army Group Mackensen (Serbia): The Army Group Mackensen (German: "Heeresgruppe Mackensen") which operated in Serbia between 18 September 1915 and 11 October 1916 during World War I under the command of Field Marshal Mackensen, was an Army Group of the German Army. <br>

Army Group Rupprecht of Bavaria: The Army Group Rupprecht of Bavaria or Army Group A (German: "Heeresgruppe Kronprinz Rupprecht von Bayern") was an Army Group of the German Army, which operated on the Western Front under command of Rupprecht, Crown Prince of Bavaria , between 28 August 1916 and 11 November 1918 during World War I.  It was formed from the short-lived Army Group Gallwitz under Max von Gallwitz (19 July - 28 August 1916).

Army Group Mackensen (Romania): The Army Group Mackensen (German: "Heeresgruppe Mackensen") which operated against Romania between 28 August 1916 and 7 May 1918 during World War I under the command of Field Marshal Mackensen, was an Army Group of the German Army.

Hans von Seeckt: Johannes Friedrich "Hans" von Seeckt (22 April 1866 – 27 December 1936) was a German military officer who served as Chief of Staff to August von Mackensen, and was a central figure in planning the victories Mackensen achieved for Germany in the east during the First World War.

Flămânda Offensive: The Flămânda Offensive (or Flămânda Maneuver), which took place during World War I between 29September and 5October 1916, was an offensive across the Danube mounted by the Romanian 2nd Army supported by Romanian coastal artillery.  Named after the hamlet of Flămânda, the battle represented a consistent effort by the Romanian Army to stop the Central Powers' southern offensive led by August von Mackensen.  The battle ended as a tactical victory for the Central Powers.

Army Group Mackensen (Poland): The Army Group Mackensen (German: "Heeresgruppe Mackensen, HGr.  Mackensen" ) which operated in Poland between 22 April 1915 and 8 September 1915 during World War I under the command of Field Marshal August von Mackensen , was an army group of the Imperial German Army.

Mackensen-class battlecruiser: The "Mackensen" class was the last class of battlecruisers to be built by Germany in World War I.  The design initially called for seven ships, but three of them were redesigned as the "Ersatz Yorck" class .  Of the four ships of the "Mackensen" class, "Mackensen", "Graf Spee", and "Prinz Eitel Friedrich" were launched, and "Fürst Bismarck" was not—but none were completed, after wartime shipbuilding priorities were redirected towards U-boat—and the ships were broken up in the early 1920s.  The lead ship of the class was named for August von Mackensen, a prominent military commander during the war.  In response to the "Mackensen"-class ships, the British Royal Navy laid down the Admiral-class battlecruiser s, all but one of which would eventually be cancelled; the sole survivor, HMS "Hood" , was completed after the end of the war.

Army Group South: Army Group South (German: "Heeresgruppe Süd" ) was the name of two German Army Groups during World War II.  It was first used in the 1939 September Campaign, along with Army Group North to invade Poland.  In the invasion of Poland Army Group South was led by Gerd von Rundstedt and his chief of staff Erich von Manstein.  Two years later, Army Group South became one of three army groups into which Germany organised their forces for Operation Barbarossa.  Army Group South's principal objective was to capture Soviet Ukraine and its capital Kiev.

Supporting paragraphs: Army Group Mackensen (Romania) August von Mackensen
Answer: Romania