Within the US, a battalion consists generally of 4 companies, but may be up to 6. A regiment consists of 3-6 battalions of the same branch (i.e. an infantry regiment, an cavalry regiment, etc.). The Army had regiments up through at least WWII, I believe, and then sliced and diced to create combined arms teams that were called combat teams (RCT) or combat commands. Post-Korea, the regimental structure was ditched and brigades were created, using 3-6 battalions of differening branches (e.g. 2 infantry battalions, 1 armor battalion, 1 field artillery battalion, 1 engineer battalion, 1 support battalion).Originally Posted by gunnut
While that's a simple and somewhat crude explanation that may be slightly off on the historical timeline, it should get the basic idea across about the difference.
Where it gets confusing is that in order to maintain the historical heritage, regiments are still used in titles, but generally don't exist (outside of 75th Ranger Regiment and the Cav Regiments). As an example, my last unit was 1st Battalion, 23rd Infantry, 3rd Brigade, 2nd Infantry Division (Stryker Brigade Combat Team). So, even though I was in a brigade, I was still linked to a Regiment, that being the 23rd Infantry Regiment. Since the 23rd Infantry Regiment HQ didn't exist, being the 1st Battalion, we had possession of all the Regimental items, to include a punch bowl made out of all the melted down CIBs and CFMBs of the men of the 23rd Infantry Regiment from Korea (they were made out of silver at the time) and the Regimental colors. Thus, the 4th Battalion, 23rd Infantry (stationed in Alaska) only had their battalion items.