In September 1862, the Battle of Antietam gave Lincoln the victory he needed to issue the Emancipation. In the battle, though the Union suffered heavier losses than the Confederates and General McClellan allowed the escape of Robert E. Lee's retreating troops, Union forces turned back a Confederate invasion of Maryland, eliminating more than a quarter of Lee's army in the process. On September 22, 1862, five days after Antietam occurred, and while living at the Soldier's Home, Lincoln called his cabinet into session and issued the Preliminary Emancipation Proclamation. According to Civil War historian James M. McPherson, Lincoln told Cabinet members that he had made a covenant with God, that if the Union drove the Confederacy out of Maryland, he would issue the Emancipation Proclamation. Lincoln had first shown an early draft of the proclamation to Vice President Hannibal Hamlin, an ardent abolitionist, who was more often kept in the dark on presidential decisions. The final proclamation was issued on January 1, 1863. Although implicitly granted authority by Congress, Lincoln used his powers as Commander-in-Chief of the Army and Navy, "as a necessary war measure" as the basis of the proclamation, rather than the equivalent of a statute enacted by Congress or a constitutional amendment. Some days after issuing the final Proclamation, Lincoln wrote to Major General John McClernand: "After the commencement of hostilities I struggled nearly a year and a half to get along without touching the "institution"; and when finally I conditionally determined to touch it, I gave a hundred days fair notice of my purpose, to all the States and people, within which time they could have turned it wholly aside, by simply again becoming good citizens of the United States. They chose to disregard it, and I made the peremptory proclamation on what appeared to me to be a military necessity. And being made, it must stand."