Following the Paris attacks in November, in which Isis (Islamic State) gunmen killed 130 people in a series of mass shootings and suicide attacks, Brussels raised its terror threat level to its highest possible alert.
The Belgian capital also went on lockdown, as police and intelligence agencies traced at least one of the Paris attackers' origins back to the Molenbeek neighbourhood of Brussels.
Roughly a week later, the Belgian Government lowered the threat level from "serious and imminent"to "possible and likely"where it has remained until the latest attacks. It is now back to its highest level.
"This is the ultimate paradox in counter-terrorism," said Bruce Hoffman, the director of Georgetown University's Security Studies Programme. "You can't protect every target, everywhere, all the time. They've been on complete alert, and still all these measures are still insufficient against a determined adversary."
The attacks hit a city where soldiers, clad in body army and Fabrique Nationale carbines were already stationed at metro entrances, government buildings and various other locations throughout the city. The soldiers often patrol in pairs and are occasionally easily marked by their large Mercedes transport trucks.