In 1820 the U.S passed the Missouri compromise that stated that slavery could not extend above the 36' 30" line. When Kansas and Nebraska were looking to join the union Stephen Douglas proposed a bill that would allow each state to vote on if they were going to be a slave state or a free state. Nebraska was far enough north that there was never really a question over whether or not it was going to be slave or free. However, Kansas was right next to Missouri, which was a slave state. This lead to a massive amount of people entering the state on the side of slavery from the south and anti-slavery from the north. Many of these people were armed leading to a number of conflicts between the two. Bleeding Kansas was not a fight between Union and Confederate troops. Rather it was fighting between pro-slavery individuals and anti-slavery individuals.

All of this happened in the lead up to the Civil War and not during the war itself. The Confederate States of America were not founded until 1860 when Abraham Lincoln was elected President and bleeding Kansas took place before them. Whenever I have heard people say that Bleeding Kansas was the first battles of the Civil War they have been referring to this point as the point that Civil War becomes inevitable. This was really the first time the U.S had had large conflicts over the matter of slavery. This was really the point where many in the nation saw that the U.S could not continue to expand and be half free and half slave. I hope this helped answer your question.