Why was the popular sovereignty a solution and then a failure?

Popular sovereignty was an idea pushed on Douglas by Southern senators in order to pass his Kansas-Nebraska Act but Douglas made it a centerpiece of his policy because he saw it as the means to unite the Democrats, North and South, behind him.  Popular sovereignty had the opposite effect: it polarized political opinion even more.  Popular sovereignty in practice touched off something very close to a civil war in Kansas as pro-slavery and anti-slavery factions organized their own state constitutional conventions and led to wave of bushwhacking and political murders.

i hope this helped

answer: secession

secession, as it applies to the outbreak of the american civil war, comprises the series of events that began on december 20, 1860, and extended through june 8 of the next year when eleven states in the lower and upper south severed their ties with the union. the first seven seceding states of the lower south set up a provisional government at montgomery, alabama. after hostilities began at fort sumter in charleston harbor on april 12, 1861, the border states of virginia, arkansas, tennessee, and north carolina joined the new government, which then moved its capital to richmond, virginia. the union was thus divided approximately on geographic lines. twenty-one northern and border states retained the style and title of the united states, while the eleven slave states adopted the nomenclature of the confederate states of america.

explanation: