Do you know where can I see the number of the available forces during the war?

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Hello. I am trying to find the amount of the forces on combat per nation and per year (and if it is possible per month). Also, I am interested in check what is the amount of soldiers on reserve, training and fighting in multiple fronts. I have read a decent amount of books but I only see this amounts sporadically. I need special focus on the years 1916, 1917 and 1918. Can you recommend something? I will greatly appreciate any reference. Thanks (in advance)!