Emancipation Proclamation in a tweet

Since others submitted 2 I'll go with 2 also.

Take #1 -

Profoundly effected the rational for the war, the wars outcome, and rights in the United States for African Americans.

Take #2 -

Freed the vast majority of slaves in the US, eventually; refocused the war from succession to slavery; kept foreign powers from interceding.

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In the second take, one could rearrange the order depending upon whether primarily concerned with civil rights, causes for the war, or winning the war. The emancipation proclamation had a profound effect on all three.

It was a key strategy in the diplomatic effort which ran parallel to the civil war to both isolate the south economically and militarily from would be european allies. That was influential on the timing of the announcement. But none of that diminishes Slavery and it's importance for why the war was fought on the Union side and Confederacy side.

The main interest of the Union was to stop secession, not the freedom of slaves, to the point that compromises were sought with the Southern states.

I don’t think that’s entirely accurate either. Maybe before the war began knowing slavery’s days were already numbered; Lincoln was willing to allow slavery to die from 1000 legislative cuts over years or decades to avoid what would become the most costly war in American lives in history. But once the war was upon him he had the opportunity to sacrifice emancipation to bring the war to an early conclusion but declined. Once war was upon him, Lincoln had paid the price and ensured personally slavery would end with the end of the war.

The Emancipation Proclamation of 1863 was but a single step in the long march for Justice. The struggle continues. 137 characters:

The 1863 Emancipation Proclamation 'all persons held as slaves… are, and henceforward shall be free' is one step in the march for Justice

133 characters to spare:

⛓ 🔫 🇺🇸 👍🏿 👍🏾 👍🏽 👍🏼

The Emancipation Proclamation dashed Confederate hopes that Europe be deceived that the War was about States' Rights instead of slavery.

135 bytes by my count.

This was the the true reason for the Proclamation, and for why it had to be issued after a victory. To be effective as a deterrent against European entry to the war it could only be played from a position of strength.

As when one's true colours are finally revealed at the conclusion of a naval ruse of war, the Union, and its governing Republican Party, unabashedly reveals with this proclamation not just its own motives but the true motives of its adversary.

In a reply to Horace Greeley of Aug. 22, 1862, Lincoln famously states:

If I could save the Union without freeing any slave, I would do it, and if I could save it by freeing all the slaves, I would do it, and if I could save it by freeing some and leaving others alone, I would also do that. What I do about Slavery and the colored race, I do because I believe it helps to save this Union, and what I forbear, I forbear because I do not believe it would help to save the Union.

Once Lincoln subsequently makes the Emancipation Proclamation, he declares fully that the one and only way to restore the Union, includes freeing the slaves.

By making the Proclamation after a victory, Lincoln also makes clear that the Union takes this step by choice; not of necessity. As a later statesman would say:

We do these things not because they are easy; but because they are hard.

Regardless of number of slaves immediately freed by the Emancipation Proclamation, it was extremely important, because it made practically impossible to save slavery. The Emancipation Proclamation greatly improved the chances of Union victory (by discouraging foreign help to Confederacy and encouraging slaves to sabotage Confederate war effort) AND ensured that in post-war Union (with rebellion states included) slavery would be reduced to marginal institution, excluded from areas where it was really profitable (most cotton states).

So, in short (90 characters):

The Emancipation Proclamation ensured post-war slavery marginal status and rapid abolition

Or, with reference to “House Divided” speech by Lincoln:

The Emancipation Proclamation irrevocably placed the slavery on a path of rapid ultimate extinction

Or, little longer but still less than 140:

The Emancipation Proclamation placed slavery on the course of rapid ultimate extinction by ensuring its post-war marginal status

UPDATE

One more version, inspired by T.E.D corrections:

The Emancipation Proclamation placed the slavery “in the course of ultimate extinction” by ensuring its post-war marginal status

140 characters on the immediate effect of the proclamation's implementation:

The Emancipation Proclamation immediately caused 20,000 slaves to be declared free in the Union occupied Carolinas on the day it took effect

Or 140 characters on the long-term effect:

The Emancipation Proclamation immediately freed many slaves and laid the path for the eventual dismantlement of slavery across the continent

The importance of the Emancipation Proclamation in 140 characters:

The Emancipation Proclamation captured the hearts & imagination of millions of Americans fundamentally transforming the character of the war

The punctuation could be better (an additional comma and full-stop for a start) but, well, it is limited to 140 characters!