Final answer:
General Anthony St. Clair's defeat in the Northwest Indian War led to the creation of the Legion of the United States, as his troops' heavy losses highlighted the need for a stronger standing army.
Step-by-step explanation:
The statement that describes events after General Anthony St. Clair led troops into Indian territory in the fall of 1791 is: C. St. Clair's defeat prompted the creation of a stronger standing army. During the Northwest Indian War, St. Clair's troops were untrained and ill-equipped, which led to them being quickly overrun by Little Turtle's Confederacy forces. This defeat was significant as some 630 officers and soldiers were killed, marking the highest casualty rate ever in an Indian war in American history. The aftermath of this defeat convinced the government that a stronger, more organized military force was necessary, which ultimately led to the formation of the Legion of the United States.