Two months after Ben Franklin helped draft the Declaration of Independence, a surprise visitor walked into his Philadelphia shop. The young man’s curly brown hair cascaded down toward his shoulders, and his English was so broken he switched to French. Thaddeus Kosciuszko, a 30-year-old Pole just off the boat from Europe via the Caribbean, introduced himself and offered to enlist as an officer in the new American nation’s army. Franklin, curious, quizzed Kosciuszko about his education: a military academy in Warsaw, studies in Paris in civil engineering, including fort building. Franklin asked him for letters of recommendation. Kosciuszko had none....