History of military uniform
A distinction should be made between uniforms and ethnic dress. If a particular people or culture favoured a distinctive dress style this could easily create the impression of uniformly dressed warriors.
The issue is further complicated by the fact that the distinctive features of particularly effective warrior classes were often copied - weapons, armor, fighting style and native dress. Thus the distinctive and colourful clothing of the Hungarian hussars became a model for hussar units all over Europe. The kilts and sporrans of Scottish highland clans were distilled into regimental dress when the British Army started to recruit from these tribal groups.
Mercenary or irregular fighters could also develop their own fashions, which set them apart from civilians, but were not really uniforms. The clothing of the German Landsknechte of the 16th century is an example of distinctive military fashion.