Historians of the war can be divided into two schools. The first considers that it was the unavoidable outcome of conflicting (51) between Northern and Southern (52) . The second blames it (53) political leaders for (54) to avert an unnecessary war. Analysts are also divided on whether the issue of slavery was the primary cause of the war, or a symptom of other, more (55) differences—especially sectional interests and the doctrine of states' rights—between the North and South which had been developing since the formation of the American republic. The fundamental (56) was economic. In the early 1840s the Northern states began the process of industrialization, modernizing their society to meet the demands of economic change. In (57) , the slogan of Abraham Lincoln's Republican party, "free labour, free land, free man" encapsulated the ideology of valuing the freedom of individuals to grasp the (58) for economic self-advancement in a booming, expanding society. The Southern states remained stubbornly (59) both economically and socially. It was a backward-looking way of life of tall white mansions on great (60) dependent on a labour system which made slaves of approximately 4 million black Americans. America was (61) divided by economic structure, and was led into fratricidal(杀同胞的) warfare by a series of political clashes. The most common cause was the future of the West. The crises over California's admission in 1850 and over Kansas-Nebraska in 1854 were (62) of the divergent economic interests of North and South in relation to the West. The North wanted free land for independent labour in the same new territories where the South (63) to perpetuate its traditional way of life by extending slavery. The issue was not the slavery already practised, but the (64) of its extension (65) the West.