At the time of the Great French Revolution of 1789, numerous people left their homes to uphold revolutionary ideas and principles they believed in. Many of them were armed not only with fists and fanatical determination, but with firearms as well.
They gathered into groups to withstand the royal army. Such groups called themselves brigades and half-brigades, though they were organized identically to government military battalions and regiments.
The unusual name was chosen to emphasize the difference between them and the regiments of royalists they were fighting against. Armed revolutionaries called themselves National Guards, though it was really the "force of commoners" which changed the outcome of the revolution.
After the glorious victory of the revolution, the National Guards were retained, received official status, and continued to exist independently of the army. Guards were used as law-enforcement units who carried out the functions of police, though many of the National Guardsmen later continued to serve as professional soldiers.
Interesting enough, that at all times, the recruiting system of National Guard was more than favorable - even a crippled old man or a youngster could join the Guards, if he was prepared to fight for the Republic and Revolution.