  == In Britain ==  My sense from a recent reading of Graves's Good-Bye to All That is that at that time brigades and regiments differed in purpose in the British army.  A regiment was a peace-time unit; officers and men were all permanently assigned to a regiment, the regiment was loosely geographically-based, and for officers, at least, there was a strong sense of identity with the regiment.  However, at this time, regiments were not a tactical unit - individual battalions served separately, and were integrated into different brigades with battalions from other regiments.  The brigade, on the other hand, was temporary, but had tactical significance - a brigade would be composed of battalions from a somewhat random assortment of regiments.  I'm not really familiar with whether this is the usual procedure, though.  Some of this might be better clarified in the article, I think.   