Final answer:
The defeat of Johnson and Dr. Grant in or near San Patricio was due to underestimating the Mexican army's strength and tactics.
Step-by-step explanation:
The mistake that helped lead to the defeat of Johnson and Dr. Grant in or near San Patricio was underestimating the strength and tactics of the Mexican army led by General Pedro de Ampudia. Johnson and Dr. Grant believed that the Mexican forces would not be able to hold their positions, but they were mistaken. As a result, their forces were unable to capture San Patricio, leading to their defeat.
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