In Britain   My sense from a recent reading of Gravess GoodBye to All That is that at that time brigades and regiments differed in purpose in the British army  A regiment was a peacetime unit officers and men were all permanently assigned to a regiment the regiment was loosely geographicallybased 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  Im not really familiar with whether this is the usual procedure though  Some of this might be better clarified in the article I think