Final answer:
The invention that brought on the decline of horse use in the United States was the automobile.
Step-by-step explanation:
The invention that brought on the decline of the use of the horse in the United States was the automobile. With the development and mass production of cars in the early 20th century, people started to rely less on horses for transportation and shifted to using automobiles instead. Cars provided faster and more efficient means of travel, making the horse-drawn wagons and carriages less practical and popular.
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