To quote Lincoln, the sentiment in play was that "government of the people, by the people, for the people, shall not perish from the earth."

The rallying cry of "Union" was about protecting the idea of Republican government. To put it in more broken down terms, the United States had conducted an election to choose a President, and everyone participated. But rather than accepting the outcome, the states which were unhappy with the result then chose to not go along with it. This is a real problem to the whole concept of an election and representative government! To quote James McPherson's characterization from his study of soldiers' motivations, many soldiers expressed preservation of the Union as a primary motivation, summed up as the fear that to do otherwise would "admit that a state can secede at will, and Republican government by majority rule would come to an end". And not only in the United States. They believed that defending Union was necessary to prove to the world that such a mode of government was viable.

Two of the letters which McPherson uses in his study provide some examples of this sentiment expressed.

I do feel that the liberty of the world is placed in our hands to defend and if we are overcome then farewell to freedom.

[If] traitors be allowed to overthrow and break asunder ties most sacred costing our forefathers long years of blood and toil, all the hope and confidence of the world in the capacity of men for self government will be lost [...] and perhaps be followed by a long night of tyranny.

In his study, McPherson cataloged 562 letters home and diaries by Union soldiers, and the above is quite emblematic of what they were feeling. A significant minority, 40 percent, wrote strongly in favor of what he categorizes as "ideological purposes such as liberty, democracy, majority rule, constitutional law and order, and survival of the Revolutionary legacy of republican government as the causes for which they fought," while a simple majority of 67 percent wrote of such matters is when including less ideological terms but nevertheless "with strong patriotic convictions".

Of interest also is that McPherson found the sentiments to cut heavily across class lines, and not just coming from the educated elite - both of the lines quotes above came from enlisted men in the ranks, not officers - so that a belief in the United States as "the beacon light of liberty & freedom to the human race", to quote one Pennsylvania Sgt., was a broadly held one which was firmly planted into Northern sentiment.

All quotes from "What They Fought For 1861-1865" by James McPherson