Why was the first battle of bull run, so shocking

You could almost say that Bull Run signaled an end to the innocence and naïveté both sides had about the war. It was the bloodiest battle in American history up to that point. There were almost 850 deaths and more than 4,000 missing or wounded. The Union defeat was a huge shock to the North, which had been sure of its massive military—and moral—advantage. Any hopes they had of a quick victory were dashed. Some in the South rejoiced over the triumph at Bull Run, which they felt proved that a Confederate soldier was the equal of any Union soldier, while others shared the feeling of most Northerners: that this was going to be a very long struggle.

Political forces often influence public opinion.