Final answer:
Colonel Leonard Wood led the First United States Volunteer Cavalry, known as the Rough Riders, formed from diverse American societal elements while excluding African Americans. Friedrich Wilhelm von Steuben also trained a diverse troop by transforming the Continental Army into a professional force during the American Revolutionary War.
Step-by-step explanation:
The individual responsible for forming a unique cavalry group made up of soldiers from different countries and who trained them in battle drill was Colonel Leonard Wood, one of the leaders of the First United States Volunteer Cavalry. This group, famously known as the Rough Riders, was composed of a diverse set of individuals, including Harvard, Yale, and Princeton graduates as well as skilled riders and shooters from the Western United States. The Rough Riders are an example of an attempt to create a military unit that represented a cross-section of American society, though it notably excluded African Americans. Their leader, Colonel Wood, alongside Theodore Roosevelt, hoped to create a capable fighting force with representation from various groups within the American populace, excluding African Americans.
Another notable figure who formed and trained a group of soldiers from various backgrounds was Friedrich Wilhelm von Steuben. Von Steuben was a Prussian military officer who played a critical role in training and disciplining the Continental Army during the American Revolutionary War. His methods and drill manual, 'Regulations for the Order and Discipline of the Troops of the United States,' had a lasting impact on American military practices.