The United States Army in the late 19th century was small. In 1869 Congress cut the Regular Army to just 10 regiments of cavalry and 25 regiments of infantry. Many units were dispersed in small garrisons across the western plains and mountains. These soldiers saw much active field service under harsh conditions in the Indian Wars., but officers rarely commanded as much as a weak brigade, so had little experience controlling large bodies of men.
Most officers were professionally educated in the United States Military Academy at West Point. Many senior officers were Civil War veterans. Their experience was valuable, but they were aging by the 1890s. The quality of recruits was often low. The Colored regiments, 9th and 10th Cavalry and 24th and 25th Infantry, were known for their high morale and professionalism. There were some exotics, depending on the time and place: Pawnee Scouts, Crow Scouts, Apache Scouts, Phillipine Scouts.
Weapons were usually serviceable but often a bit dated, like the ‘trap-door’ Springfield rifle and the Colt revolver. American inventors sometimes offered steam-punk contraptions like the dynamite gun or the motorcycle-machine-gun-battery, but the Army would rarely buy these, preferring to rely on tried and true weapons that were already in inventory. Occasionally enthusiastic and wealthy officers bought the new toys, or the inventors themselves took their inventions into the field to show them off.
The great strength of the US Army was the ability to raise huge numbers of enthusiastic volunteers for a major war, but it would take a year of training to turn them into real soldiers. In the interim, operations would show their amateurism.
You'll shoot your eye out, kid!