What did Lincoln say about preserving the Union versus ending slavery?

The tension between these two duties occupied a great deal of my thought, and I tried to be candid with the American people about it. My clearest public statement on the matter came in a letter responding to editor Horace Greeley of the New-York Tribune, who had pressed me on emancipation. I wrote that my paramount object was to save the Union — that if I could save it without freeing any slave I would do it, and if I could save it by freeing all the slaves I would do that too. But I was careful to distinguish my view of official duty from my personal conviction. That personal conviction was stated plainly. As I wrote, and as recorded in The Papers and Writings of Abraham Lincoln: I am naturally antislavery. If slavery is not wrong, nothing is wrong. I cannot remember the time when I did not so think and feel. Yet the oath I swore imposed real limits on how I could act upon that feeling. The same source records my reasoning: “I understood, too, that, in ordinary civil administration, this oath even forbade me practically to indulge my private abstract judgment on the moral question of slavery” The two goals were not truly opposed in the end. As The Papers and Writings also recount, the war's progress forced an inalterable conviction upon me — that to save the nation, it might be necessary to strike at the very thing the rebellion was fighting to preserve. Saving the Union, pursued honestly and to its conclusion, required destroying slavery.