Fatherland Volunteers is a 
Brazilian decision. It requires that Brazil is at war, and have researched Army Decision Making.
It makes Brazil lose 1 War exhaustion and gain 10 prestige. It can be a help in a long dragged out war, but it is best to safe it until Brazil actually have a problematic level of war exhaustion.
Ingame description
During The War of The Triple Alliance, when Paraguayan troops invaded the Brazilian province of Mato Grosso, Pedro II, the Brazilian Emperor, angered by what he saw as incompetence on part of the Brazilian military, demanded that he himself be allowed to go in person to the front. When both the Cabinet and the National Assembly refused his wish, Pedro threatened to abdicate the imperial throne and join the army as a volunteer, reportedly saying that 'If they can prevent me from going as an Emperor, they cannot prevent me from abdicating and going as a Fatherland Volunteer'. Naturally, the Cabinet was left without much choice. Brazilians who later volunteered during the war were called 'Fatherland Volunteers', in memory of Pedro's quote. Say it like Pedro did, and the nation will follow wherever you lead.