The inclusion of pro-slavery Missouri representatives would give the pro-slavery party a Congressional majority, in addition to the broader moral question raised by the growth of slavery. The Missouri Compromise was the result of a series of compromises reached by Congress.
Although many similar moments of importance transpired throughout the war years, I would argue that the summer of 1862 and especially the lynchpin Union victory at Antietam left the greatest impact on the war's course, setting in motion events that empowered the liberation of an enslaved race and ensured the young
