Why did the First Battle of Bull Run lead the Union to change its strategy?

The Union expected a quick, easy victory, but its defeat at Bull Run showed that the Confederacy was stronger and the war would likely be long. The Union therefore had to prepare for a more sustained campaign. Choice A, “The Union realized that victory would not be as easy as it originally thought,” captures this lesson; Bull Run did not end the war or prove Richmond could never be captured.

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