A "volunteer army" refers to a military force where the soldiers willingly choose to join and serve without being compelled or forced to do so. Full definition
Grant Shapps's volunteer army of supporters are mobilised and are touring the doors making their pitch in each and every marginal. (totalpolitics.com)
And now that the Bundeswehr was reformed into a pure volunteer army, it has trouble getting onto its current nominal manpower,... (politics.stackexchange.com)
I would like to make some corrections to your article on drug trials carried out by the US Department of Defense on volunteer army personnel (3 August, p 6). (newscientist.com)