Cercar vs. sitiar vs. assediar
While assediar, cercar, and sitiar can all translate to to besiege or to surround, they represent different tactics and intensities. Cercar is the most general term for physical surrounding, sitiar is the specific military term for a prolonged blockade, and assediar implies persistent attacks, pressure, or harassment.
Cercar
B1This is the most common and neutral word meaning to surround, to encircle, or to close in. It describes the physical act of forming a perimeter around a person, place, or object to prevent escape or entry, but it does not necessarily imply aggressive combat or a long duration.
A polícia decidiu cercar o quarteirão para capturar o fugitivo.
(The police decided to surround the block to capture the fugitive.)
Vamos cercar o jardim com uma grade nova.
(We are going to fence in the garden with a new railing.)
Os lobos começaram a cercar a presa antes do ataque.
(The wolves began to circle the prey before the attack.)
Sitiar
C1This word specifically means to lay siege in a military or strategic context. It implies surrounding a fortified place (like a city or castle) and cutting off supplies, communications, and reinforcements to force a surrender through isolation or starvation.
O general ordenou às tropas que fossem sitiar a capital inimiga durante o inverno.
(The general ordered the troops to lay siege to the enemy capital during the winter.)
Eles tentaram sitiar o castelo, mas as muralhas eram muito altas.
(They tried to besiege the castle, but the walls were too high.)
O exército conseguiu sitiar a região e bloquear todas as estradas.
(The army managed to lay siege to the region and block all the roads.)
Assediar
B2While this can mean to besiege in a military sense (involving constant attacks rather than just waiting), it is most commonly used today to mean to harass, to mob, or to pester. It conveys a sense of relentless, annoying, or aggressive pressure, whether from an army, a crowd of fans, or a bully.
Os canhões continuaram a assediar a fortaleza dia e noite.
(The cannons continued to besiege the fortress day and night.)
Os paparazzis costumam assediar os atores famosos no aeroporto.
(Paparazzi often mob famous actors at the airport.)
É inaceitável assediar um funcionário no ambiente de trabalho.
(It is unacceptable to harass an employee in the workplace.)
Summary
In summary, use cercar for the general physical act of surrounding or fencing something in. Use sitiar strictly for military blockades designed to isolate a location. Use assediar for situations involving constant pressure, unrelenting attacks, or psychological harassment.