"To bell a cat" is a phrase which means "Doing an impossible task". I need to translate this phrase into Spanish to narrate a story.
I guess that the rationale of the phrase is that a cat wouldn't let you put a bell on it. There is a similar expression in Spanish meaning something similar:
Poner una pica en Flandes.
pica is a spear-like weapon.
Flandes is a designation for the Netherlands. The sentence conveys how difficult it was for Carlos V to get his soldiers to the Netherlands (he didn't have good routes, neither by land or sea, since he was engaged in many wars with different countries).