This is a common question, and we have several answers in our FAQ section - this one by /u/Georgy_K_Zhukov is a good, comprehensive overview of the topic and some of the reasons for the clashing popular narratives. The summary is that historians absolutely assert that slavery was the driving cause behind the secession of the Confederacy and the resulting conflict; assertions to the contrary are at best arguments from ignorance, and at worst intentional efforts to distort the historical record in the pursuit of an often racially prejudiced political or moral agenda. Mississippi's own declaration of secession is famously prefaced with the phrase "our position is thoroughly identified with the institution of slavery", and other declarations contain similar references to slavery as a motivating factor. The common assertion that the war was actually about states' rights is completely inconsistent with Southern political strategy pre-war, which very much expected the federal government to undermine the autonomy of other states in order to protect and support the institution of slavery (like in requiring them under federal law to deal with fugitive slaves even if slavery was illegal within their borders). Do feel free to ask additional questions if you would like to know more, but Zhukov's linked answer should cover a lot of the common ones.