What was the primary and overriding cause of the civil war?

There were many different Causes of the Civil War between the Union of the North and the Confederacy of the South. The two major causes of the Civil War were slavery and States Rights. The economy in the south was based on the cash crop plantation system which depended on the institution of slavery whereas the economy in the north was based on manufacturing and industry and was not reliant on slaves. The south were concerned about the dominance of the northern states following the election of Abraham Lincoln and the Republican party and the possible infringement of states rights.

why was the national assembly formed by the third estate?

the estates-general had not been assembled since 1614, and its deputies drew up long lists of grievances and called for sweeping political and social reforms. the third estate, which had the most representatives, declared itself the national assembly and took an oath to force a new constitution on the king.

the lowest among the social higher rank were the working class. this class remained detached to the political progression   of the country and was hostile to the other two classes