The army was the right wing of the forces that invaded France and Belgium as part of the Schlieffen Plan offensive in August 1914.

On 5 September, when the French 6th Army launched its surprise attack on Ourcq, a tributary of River Marne, they were brought under heavy fire by German guns.

As darkness fell, von Gronau knew that he had won the necessary time to save the 1st Army from a surprise attack and retreated.

[2] In September 1915, Gronau was appointed to command of XXXXI Reserve Corps, swapping places with Generalleutnant Arnold von Winckler.

His oldest son was the flight pioneer Wolfgang von Gronau who crossed the Atlantic Ocean from East to West flying a Dornier Wal D-1422 landing in the Hudson River on 26 August 1930.