Question: During the First World War, the 3rd Cavalry Brigade of the British Army served on the Western Front, but where did the German Army first attack them in that main theater of the war?

Context paragraphs:
3rd (Ambala) Cavalry Brigade: The Ambala Cavalry Brigade was a cavalry brigade of the British Indian Army formed in 1904 as a result of the Kitchener Reforms.  It was mobilized as 3rd (Ambala) Cavalry Brigade at the outbreak of the First World War as part of the 1st Indian Cavalry Division and departed for France.  It served on the Western Front with the 1st and 2nd Indian Cavalry Divisions until it was broken up in March 1918.

8th Cavalry Brigade (United Kingdom): The 8th Cavalry Brigade was a cavalry brigade of the British Army in World War I.  It was formed in Belgium in 1914 and served on the Western Front as part of the 3rd Cavalry Division.  It left the 3rd Cavalry Division on 14 March 1918.

3rd (Meerut) Cavalry Brigade: The 4th (Meerut) Cavalry Brigade was a cavalry brigade of the British Indian Army that formed part of the Indian Army during the First World War.  It was formed as 14th (Meerut) Cavalry Brigade in November 1914 to replace the original Meerut Cavalry Brigade that had been mobilized as the 7th (Meerut) Cavalry Brigade for service on the Western Front.  It remained in India throughout the war, before taking part in the Third Anglo-Afghan War in 1919.

4th Cavalry Brigade (United Kingdom): The 4th Cavalry Brigade was a cavalry brigade of the British Army.  It served in the Napoleonic Wars (notably at the Battle of Waterloo), in the First World War on the Western Front where it was initially assigned to The Cavalry Division before spending most of the war with the 2nd Cavalry Division, and with the 1st Cavalry Division during World War II.

6th Cavalry Brigade (United Kingdom): The 6th Cavalry Brigade was a cavalry brigade of the British Army.  It served in the Napoleonic Wars (notably at the Battle of Waterloo), in the First World War on the Western Front where it was assigned to the 3rd Cavalry Division, and with the 1st Cavalry Division during World War II.

3rd Cavalry: 3rd Cavalry, 3rd Cavalry Division, 3rd Cavalry Brigade or 3rd Cavalry Regiment may refer to:

7th Cavalry Brigade (United Kingdom): The 7th Cavalry Brigade was a cavalry brigade of the British Army.  It served in the Napoleonic Wars, notably at the Battle of Waterloo.  It was reformed in 1914 and served on the Western Front as part of the 3rd Cavalry Division until the end of World War I.

3rd Cavalry Brigade (United Kingdom): The 3rd Cavalry Brigade was a cavalry brigade of the British Army.  It served in the Napoleonic Wars (notably at the Battle of Waterloo), in the Boer War, and in the First World War on the Western Front where it was initially assigned to The Cavalry Division before spending most of the war with the 2nd Cavalry Division.

4th (Secunderabad) Cavalry Brigade: The 4th (Secunderabad) Cavalry Brigade was a cavalry brigade of the British Indian Army that formed part of the Indian Army during the First World War.  It was formed as Secunderabad Cavalry Brigade in February 1915 to replace the original brigade that had been mobilized as the 9th (Secunderabad) Cavalry Brigade for service on the Western Front.  It remained in India throughout the war.

Western Front (World War I): The Western Front was the main theatre of war during the First World War.  Following the outbreak of war in August 1914, the German Army opened the Western Front by invading Luxembourg and Belgium, then gaining military control of important industrial regions in France.  The tide of the advance was dramatically turned with the Battle of the Marne.  Following the Race to the Sea, both sides dug in along a meandering line of fortified trenches, stretching from the North Sea to the Swiss frontier with France, which changed little except during early 1917 and in 1918.

Supporting paragraphs: 3rd Cavalry Brigade (United Kingdom) Western Front (World War I)
Answer: Luxembourg and Belgium