I am indebted, again, to Michael Bully of the Great War Forum for suggesting that my post regarding Finland need a little more research.
Finland was annexed by Russia before the First World War. During the First World War more than a thousand Finnish officers and NCOs went to Germany secretly and joined the 27th Royal Prussian Jager Battalion.
By the time WW1 broke out, the German Army was organised into an Army Crops, most of which was under the command of the Prussian Army.
Thank you Michael.