Why did Lincoln wait to issue the Declaration of Emancipation?
(Apologies in advance: not being American, while I've read up somewhat about the military aspects of the Civil War, I am somewhat unclear as to its political dimension.)
Lincoln did not declare the slaves free until 1863, despite his personal beliefs.
Did he wait because he was not sure of the political support in the North for accepting Abolition as a war aim, along with stopping Secession? Or did he wait to preserve the possibility of negotiations with the Confederacy? If it was the second reason, is there any indication of what he was willing to compromise on? If the first, what changed?
In the United States the most reliable way to end slavery was a constitutional amendment. This takes the most political consensus (2/3rds of the house and Senate and then 3/4th of the states) and is thus the most difficult to implement and reverse.
The Thirteenth amendment was first introduced Dec 1863, and failed to pass in the house on the first vote June 15, 1864 by 13 votes. Lincoln organized a pretty masterful political offensive involving government jobs for outgoing congressmen in a lame duck congress in order to finally pass the amendment through the House which was the most difficult stage of the process. Still it took years and right up until the final vote in congress, passing it was not a forgone conclusion. Lincoln passed the first part of these hurdles during the Civil war( Apr 12, 1861 - Apr 9, 1865) with the ratification of of the 13th Amendment by the senate(April 8, 1864) followed by the house 9 months latter (January 31, 1865). Ratification by 3/4ths of individual states after the war could be relied upon as the Southern secessionist governments would be controlled by the Union during reconstruction. As predicted the states achieved the 3/4th's majority to ratify it after the end of the War, Dec 6, 1865.
The Emancipation proclamation wasn't about freeing the slaves. At it's best it was temporary war time fiat which only affected the slaves in the secessionist states outside of the Unions control. The evening before Lincoln signed the Proclamation Robert E Lee and his entire army were in Northern Territory, The Union Armies having been repulsed by Lee from Virginia earlier that year.
The Emancipation proclamation was a political vehicle, as was the timing. Lincoln was looking to redefine the war. He had previously defined the war as a war to preserve the Union, now Lincoln would telegraph ending slavery as a major war goal. Through this redefinition Lincoln hoped to block Britain and France from getting involved. These countries who were considering recognizing the confederacy (Britain and France) were popularly against slavery. By making the American Civil War about slavery Lincoln targeted these European nations making recognition of the south politically untenable for them domestically.
So Lincoln knew he needed a constitutional Amendment to end slavery in a meaningful way. Why did he sign the Presidential order the Emancipation Proclamation
in Jan 1, 1863, knowing the next President could simply repeal Lincolns order? Or after the war the supreme court could nullify his war powers act and rule his proclamation unconstitutional. In the absence of a change to the Constitution, any act taken by President Lincoln would not survive much beyond his presidency.
Lincoln did it to preserve the union. Simple put the American Civil war was fought on many fronts. Economic, Political, Military, and Diplomatic. It was the diplomatic war which needed Lincoln's action on Jan of 1863. Britain and France were threatening to enter the war on behalf of the Confederate States. Lincoln used the Emancipation Proclamation to reframe the war from one about preserving the Union, into a war to end slavery. That's what the emancipation proclamation did. It announced that ending slavery would be a major goal of the Union in the War. Lincoln knew if the war was framed as one against slavery it would make Britain and France's intervention untenable giving both countries had already abolished slavery Britain(Slavery Abolition Act in 1833) and France (Louis X abolished slavery in 1315, although slavery continued in French Colonies until 1848).
The timing was dictated by politics, and as previous answers said more precisely by a Union victory at Battle of Antietam. Technically Antietam wasn't a victory for the Union it was a stalemate. But after months and months of seeing his armies retreating from the gates of Richmond, seeing Lee retreat from Maryland back into Virginia was represented in the Northern Newspapers as a victory. So Lincoln used that "victory" to roll out his diplomatic offensive against Britain and French intervention.
As Lincoln had hoped, the Proclamation turned foreign popular opinion in favor of the Union by gaining the support of anti-slavery countries and countries that had already abolished slavery (especially the developed countries in Europe such as Great Britain or France). This shift ended the Confederacy's hopes of gaining official recognition.
Since the Emancipation Proclamation made the eradication of slavery an explicit Union war goal, it linked support for the South to support for slavery. Public opinion in Britain would not tolerate direct support for slavery. British companies, however, continued to build and operate blockade runners for the South. As Henry Adams noted, "The Emancipation Proclamation has done more for us than all our former victories and all our diplomacy." In Italy, Giuseppe Garibaldi hailed Lincoln as "the heir of the aspirations of John Brown". On August 6, 1863, Garibaldi wrote to Lincoln: "Posterity will call you the great emancipator, a more enviable title than any crown could be, and greater than any merely mundane treasure".
Mayor Abel Haywood, a representative for workers from Manchester, England, wrote to Lincoln saying, "We joyfully honor you for many decisive steps toward practically exemplifying your belief in the words of your great founders: 'All men are created free and equal.'" The Emancipation Proclamation served to ease tensions with Europe over the North's conduct of the war, and combined with the recent failed Southern offensive at Antietam, to cut off any practical chance for the Confederacy to receive British support in the war.