Maestre de campo was a rank created in 1534 by the Emperor Charles I of Spain, inferior in rank only to the capitán general and acted as a chief of staff.
Their personal guard consisted of eight German halberdiers, paid by the king, who accompanied them everywhere.
Immediately inferior in the chain of command was the sargento mayor.
One of the most famous maestre de campos was Julian Romero, a common soldier who reached that rank and that brought victory to the Spanish tercios at the battles of San Quintín and Gravelines.
In the overseas colonies of the Spanish Empire a governor held the rank of capitán general over his local forces and would appoint his maestre de campo.