Technology has changed the way in which wars are planned and fought, but history suggests that there is no magic formula for determining how many troops the United States needs to prevail in battle.
The bulk of the coalition ground forces on hand for the start of the war on Iraq consisted of four divisions: the Army's 3rd Infantry and 101st Airborne Divisions, the 1st Marine Division, and the British 1st Armored Division. The mission of this force was to seize southern Iraq up to Baghdad itself.
In the 1991 Persian Gulf War, the mission was much smaller -- to retake Kuwait and the southeastern corner of Iraq. Yet the number of U.S. divisions dedicated to this much more limited task was nine. With British, French and Arab forces added, the number of divisions or division equivalents exceeded 13, more than three times as many as began the current Iraq war. As a result, the ground combat force in the 1991 war was able to form a fairly solid front several hundred miles long across the northern Saudi Arabian border, and then pinwheel to the north and east through southeastern Iraq and Kuwait.
Given the amount of territory they have had to cover in the current war, U.S. and British ground forces have only been able to establish a very porous, limited line in their advance to the north. That contributed to supply problems early last week but did not halt the advance to Baghdad by week's end.
In the Vietnam war, the United States covered an even more difficult terrain with forces that were numerically inadequate. South Vietnam is approximately 700 miles long and 100 miles wide. Moreover, it has a long open border on the west with Cambodia and Laos. Yet the United States battled the Viet Cong and the North Vietnamese army with, at the high point, just seven full divisions and perhaps an additional two division equivalents. Forming a line and sweeping across or up South Vietnam was out of the question. Almost by default, the strategy became one of wandering the mountains, swamps and jungles hoping to trap the enemy. More often than not, the enemy found an unguarded avenue of escape.
During World War II, large-scale ground operations consisted of forming lines and sweeping forward. In some cases, concentrated forces were needed in small areas. In the major battle with Japan in 1944 over Saipan, a 5-mile-wide and 12-mile-long island in the western Pacific, the United States used three divisions -- two Marine and one Army. That worked out to one division for every 20 square miles, versus one division for every 8,000 square miles in Vietnam. That ratio might help explain why victory in Vietnam was elusive.
But the need for troops depends on the size, skill and spirit of U.S. foes. In the end, the answer to the question of how many ground forces are needed in a given war may be: however many it takes.