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To stop foreign support from aiding the Southern war effort and make the war about slavery.
Emancipation Proclamation made by Abraham Lincoln
When Lincoln announced the emancipation proclamation, some of the slaves were free which then aided the union army to end the war.
The Emancipation Proclmation freed slaves in territors not already under Union control. The Gettysburg Address was to sustain public support for the war effort.
Contrary to popular belief, the Emancipation Proclamation's goal was not really to free slaves. It stated that all slaves in the 10 rebelling states were free. You see the problem here? With no control over the Confederacy, the Emancipation Proclamation had little effect on the slaves in the South. The Proclamation made freeing slaves an explicit goal of the Union war effort, and was a step towards outlawing slavery and conferring full citizenship upon ex-slaves.
Britain and France had to drop their plans to help the Confederates, as the war had now been turned into an official crusade against slavery.
By keeping the British and the French from sending aid to the Confederacy. Both those countries had abolished slavery years before, and could not be seen to fight against the (newly declared) liberators of the slaves.
To save his country, his presedency, and in an effort to stop war
The Preliminary Emancipation Proclamation seemed to be US President' Lincoln's position on the final view of slavery in the US. But it was a war measure meant to help the Union war effort. Most Americans in the North were not sure exactly what happen when the Preliminary Proclamation was made. There was no groundswell of anti-slavery sentiment in the North in 1862.
After defeating General Lee first attempt to invade the North (Gettysburg would be the second and last attempt), President Lincoln (who waited for a Union military victory) announced the Emancipation Proclamation, the beginning of the end of slavery in the USA. President Lincoln waited for a Union victory, to give the document some credibility. Announcing the Proclamation after a lot of defeats, would be a wasted effort.
The Proclamation of 1763 was a royal proclamation and was signed by King George III. It was an effort on the King's part to claim land in North America as British territory.
Because they were losing all their battles in Virginia, and it would have looked like a desperate measure.