The first major battle of the Civil War is at First Bull Run at Manassas, VA. Union forces of 38,000 men led by Brig. Irvin McDowell attack 32,000 Confederate forces led by Brig General P.G.T. Beauregard. The confederates successfully counterattack and push McDowell's forces back to nearby Washington. Total casualties in the battle are close to 5,000, and huge caches of equipment are lost by the retreating Union forces. The battle forces the Union Army to reexamine its officer corps and come up with new battle tactics, although it takes years for Civil War generals to appreciate the deadly effects of new rifle and cannon technology.

The First Division under command of Major General John Pershing fights the first major American battle of World War I against the German 18th Army at Cantigny, France in 1918. In a three day see-saw battle with experienced German troops, American forces suffer heavy casualties. The army learns hard lessons about how to fight open trench, long range artillery warfare with infantry formations.

The Army suffers initial defeats in battles such as Kassarine Pass in North Africa and in the Philippines. The Army soon rebounds with a huge influx of men and materiel as it perfects the combined use of mobile armored units and airpower. This turns the tide against the Germans and Japanese in major operations in Africa, Europe and the Pacific that ultimately lead to their surrender in 1945. The Army also supervises the Manhattan Project and the building of the atomic bomb, leading to a new era in modern warfare. US forces killed, wounded or missing in action during World War II total over 1 million.

The first US Army battle of World War II takes place in Tunisia between Major General Orlando Ward, who commands the 1st Armored Division, and German General Erwin Rommel's 10th and 21st Panzer divisions and the Africa Korps. American forces are routed in a series of German attacks and forced to retreat through Kassarine Pass. American forces have more than 3,000 casualties, 3,000 soldiers listed as missing, and lose hundreds of tanks, artillery pieces and large amounts of supplies. They prove to be inexperienced and overmatched by their German counterparts, forcing American commanders to reexamine training and battlefield tactics and reorganize the army's division structures.

The first major battle of the Vietnam War between US forces and North Vietnamese regulars involves the 1st Cavalry Division (Airborne) and the 32, 33, and 66th North Vietnamese Regiments in the Ia Drang Valley in 1965. American forces attempt to stop a North Vietnamese offensive by helicoptering in 2,700 men in quick strike tactics. The battle lasts for several days and involves bloody hand to hand combat. Although American forces kill 10 times as many of the more numerous North Vietnamese forces, who suffer as many as 4,000 casualties, several American units are decimated. Both sides claim victory.

Partly in response to the Army's failure to deploy its Apache attack helicopters in the war over Kosovo, newly appointed Chief of Staff General Eric Shinseki announces a new Army transformation effort to quickly develop a medium weight force to deploy anywhere in the world in 96 hours. He begins efforts to train two new brigades at Ft Lewis, Washington, using wheeled vehicles instead of traditional heavier tracked armored personnel carriers. His plan is to sustain current Army forces and build new medium weight forces, while aggressively designing a new high tech Army called the "objective force" that will replace the current armored forces by 2012-2015.