Anyone watching Iraq war coverage has seen a stream of numbers go by, identifying particular Army divisions—the 101st Airborne, the 3rd Infantry, etc. What do these numbers mean? And if there's a 101st Airborne, what happened to the 100th and 102nd?
The first thing to know is that the Army's divisions were numbered in the order they were created. So the 1st Division was actually the first division; then came the 2nd, 3rd, etc.
There are, of course, gaps in the sequence. Today's Army has eight infantry divisions: the 1st, 2nd, 3rd, and 4th, along with the 10th, 25th, 82nd, and 101st. What happened to the rest of them? Well, the military has cyclically expanded in wartime, creating lots of new units—during World War II, for example, the Army's had infantry divisions running all the way up to the 106th. But during peacetime, most of the war units are deactivated, which accounts for the holes.
How does the Army pick which divisions to keep? Each unit has its own customs and history, and the Army basically preserves the ones with the most glorious lineage. Take the 101st Airborne Division, which has been part of the Army since 1942. During World War II, the "Screaming Eagles" parachuted into Normandy and fought their way across Europe, making a heroic stand at Bastogne during the Battle of the Bulge. The Army has kept the division on active duty ever since. During the same war, the Army's 100th and 102nd Divisions served no less bravely but somewhat less famously. Both were shuttered for good after the war.
TV coverage of Gulf War II also refers to various Army regiments (notably the 7th Cavalry Regiment, which has already tangled with Iraqi forces); but forget about trying to understand that numbering system. For a while, the Army issued regimental numbers in sequence. But the system gave out during the Civil War, when states raised and numbered their own regiments, and became further muddled during World War I, when newly formed federal regiments tried to reclaim the numbers of their Civil War forebears.
To make things more confusing, the Army has a habit of combining many regiments in the same unit. The 1st Brigade, 3rd Infantry Division, for example, includes parts of the 7th Infantry and 69th Armor regiments. It also includes a number of support units, whose numbers often bear little or no relation to the number of the combat unit they support.
Bonus Explainer: Division, regiment, battalion—what's the difference? Divisions have 10,000-to-15,000 soldiers divided in three-to-five combat regiments and a number of support units. Regiments have 3,000-to-5,000 soldiers and include several combat and support battalions. Each battalion has three-to-five line companies of 100-to-150 soldiers apiece. Companies break down into three-to-five platoons of 20-to-40 people, which in turn break down into squads of eight-to-12.