The allies
France had 21 active army corps (one army corps included approx. 40,000 men), 22 reserve divisions, four territorial brigades, 10 cavalry divisions, which were joined, during the August, by two divisions from Algeria, one division from Morocco, one alpine division and five alpine groups. This amounted to a total of 34 army corps and 10 cavalry divisions.
The British Expeditionary Force consisted of two army corps.
Belgium had six infantry divisions.
The allied forces amounted to a total of 39 army corps and 12 cavalry divisions.
Please note:
The infantry consisted of foot soldier units. They advanced through fields and forests, almost by night and day, covering 30-50 km per 24 hours, while carrying their rifles and a 30kg rucksack under the blazing sun, during the heat wave in summer 1914.

The German empire
The imperial army was created from armies from the different kingdoms/principalities and consisted of 21 active army corps, 13 reserve corps, 17 mixed reserve brigades ("Landwehr"), which were equivalent to four army corps, 33 Landwehr brigades (equivalent to eight army corps), 10 cavalry divisions, followed by five and a half army corps and a division of naval fusiliers from 5 October 1914.
The German forces represented a total of 52 army corps and 10 cavalry divisions or approx. 1,600,000 on the front line.

Distribution of allied forces at the start of the war
The French army represented approx. 1,100,000 men.
The British Expeditionary Force, which was commanded by General French and set up its base on the Sambre, west of Avesnes-sur-Helpe, lined up 70,000 men on the front who had only just arrived. It consisted of 1,600,000 soldiers in 1915.
The six Belgian divisions added up to 95,000 men. The 1st and 2nd divisions each had 18,500 men, while the other divisions had 14,500 men.
The combined allied forces had 1,265,000 soldiers at the end of August 1914, which was approx. 335,000 men less than the German armies.