[Mobhack]

A regiment is a collection of batallions.

In some armies, the regiment is the deployable unit (the continental or unitary regimental system) and in others the batallions are the deployable units (e.g. the British system). The latter seems to be taking over from the unitary system worldwide.

In some armies, the regiment (a large one of 4 batallions) takes the place of the brigade, some armies deploy 2 regiments to form a brigade, and the British system appoints 3-4 batallions (not often ones from the same parent regiment) to a brigade. The US system of WW2 was to add divisional support units to a regiment to form a Regimental Combat Team - thier equivalent of a brigade.