Fighting to end slavery didn't become an issue until later in the war. If it had been an issue from the beginning, it wouldn't have taken Lincoln until 1863 to authorize the formation of the first two black regiments in the north, the 54th and 55th Massachusetts Regiments. Yes, there were contraband regiments organized in the South using freed slaves. The 54th and 55th consisted mostly of free men, who had been born in the North. But some of the men who joined those two units were escaped slaves, and they had to lie, and claim they were free, when they took the oath to enlist.
If you asked the men in uniform why they were fighting back then, the Union men would say "to hold the Union together." The Confederate men would say: "States' Rights."
The following letter was widely published in several of the newspapers of New York State in late 1864. It was discovered in the 10/15/1864 issue of the Poughkeepsie Telegraph:
A Union Soldier's Letter
We have been favored by an old resident of Mabbettsville, in this county with the following interesting letter written to him by a nephew in the army, dated Morris Island, S.C., September 24, 1864:
Dear Uncle:--Your good advice I will try and follow. I tell you, George B. McClellan is the only man, that can carry the old ship of State safely through; already we are drifting near the rock that will submerge the noble ship, and we need a man at the helm that will take her out into the broad ocean and guide her toward and into the port of Peace. I say there is too much negro about this matter; only look at the thousands of valuable lives that have been sacrificed for the black man, but my opinion is the South are not fighting for slavery now, but for their honor; but the present administration are continually harping on the negro. They say we are determined to break the bonds of every slave--or disunion God forbid I should ever have those feelings. No, no. The Union must and shall be preserved. Let the negro go. The white man must rule and reign.
The noble and tried patriot to-day stands before the American people for the high position of President of these United States. His enemies will ask you what he has ever done to entitle him to occupy the presidential chair? He has done much. Why did he not do more? Simply because he was never supported by the Administration as he should have been; troops were withheld from him, when he called loudly for them. The great secret was, he was too popular with the people and soldier. The Republicans were afraid of him. But thank God he is as much beloved to-day as ever.
The soldiers love him, and when their votes are counted you will find we will roll-up such a majority for General George B. McClellan that will astonish the country. He is our choice, and if you could have witnessed as I did the scene that transpired when he was relieved from command, it would have made your heart (though it were adament) melt to see the tears trickle down the cheek of the war worn veteran and the raw recruit when the news reached them, but I trust the day of deliverance is at hand.
Dear Uncle, though you may have never engaged in politics before in your life, I implore you to put your shoulder to the wheel, and every chance you have don't neglect the opportunity of urging the claim of Little Mac upon your friends. Please tell them to stand by him. I hope Old Duchess [county] will roll up a large majority for him. I must close as it is near 10 o'clock at night. Please write me a few lines. Your nephew,
Edwin A. Hoag.
And how did it work out for McClellan?