I have not played 1.5 yet (currently working on my 12th CoW game). In CoW, manpower seems to be a function of the population available in the provinces of a nation. Normally the manpower total in a typical CoW scenario is pretty much fixed (e.g. 39 map), because the scenario takes place over a period of a few years- not generations as in Civilization, where you expand a civilization's/nation's territories and settlements/cities over a period of 6,000 years.
I understand the principle of manpower been required for all buildings and materials. After all, people are the most important asset of any nation- but abundance of resources and cash always helps. So, what are the options to increase manpower, which is an issue brought in this thread by various folks? ...