The small independent country of Hohenzollern in southwestern Germany was run by two princes until 1850, when it was given away to the king of Prussia, who was a distant cousin to the princes (also descending from the Hohenzollern family).
Prussia expanded mainly with war, but in this particular case, Wikipedia does not say why the country of Hohenzollern was given away to Prussia in the German, French or English versions.
Since this happened before the war of 1866 (where Prussia basically took over most of Germany by defeating Austria), I do not see why the Hohenzollern princes would do such a thing as losing their independence.