The word militaban is the imperfect form of militar in the third person plural.
See the full militar conjugation

militar

¿Qué efecto tuvo eso en los que militaban contra la guerra y otras cosas?
What was the effect of that on people who were against the war, and taking up different issues?
Tampoco tuvo en cuenta que varios miembros destacados del partido que militaban activamente durante el mismo período que E. N. K.
Nor did it take into account the fact that several prominent members who were active during the same time as E.N.K.
¿No tendrán que ver estas declaraciones con la manera en que estigmatizó el presidente a la Comunidad de Paz de San José de Apartadó cuando dijo que varios de sus líderes militaban en las filas de las FARC?
Could these statements have anything to do with the way that the president stigmatized the Peace Community of San José de Apartadó when he said that several of its leaders served in the ranks of the FARC?
Los trabajadores en servidumbre y las personas que militaban por su liberación estimaban que la liberación de pequeños grupos de personas no podía en ningún caso ser una solución viable para un problema que debía tratarse a escala nacional de manera sistemática y global.
Both the bonded labourers themselves and the persons trying to obtain their release felt that freeing small groups of individuals was in no way a viable solution to a problem which had to be dealt with systematically and comprehensively at a national level.
A la edad de 23 años, Bernardo Arévalo Padrón se enroló como policía de la Seguridad del Estado, con la misión de vigilar a quienes militaban en favor de los derechos humanos.
When Bernardo Arévalo Padrón was 23 years old, he joined the State Security police. It was his duty to maintain human rights activists under surveillance.
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