Wars ultimately come down to manpower and, from this, the portion of the population that is actually able to assist in the war effort (able-bodied souls for fighting, production, etc...). Manpower Fit for Military Service is used by the GFP final ranking to further refine a nation's overall military strength.
Manpower Fit for Military Service data through 2015.
Sources: CIA.gov, CIA World Factbook, wikipedia.com, public domain print and media sources and user contributions. Some values may be estimated when official sources are lacking.