It is an interesting article but ... I think it starts well and then loses itself immediately.

What Follows is MY Opinion ... and, surprisingly to me, Mostly ONLY MY Opinion ...

From the days before there is any known or reliable history, there were assuredly “slaves”. That is, “slaves” in the classical sense.

This Definition will do for the topic, from Mirriam-Webster; Definition of slave
1 : a person held in servitude as the chattel of another
2 : one that is completely subservient to a dominating influence

It is my contention that, since ALL “Regular People” were beholden ... subservient ... owed their continued existence to Their Kings or Nobility Structure, then ALL “Regular People” were, in fact, slaves.
The Greeks had “actual slaves” but, unless you were part of the “Nobility” you were simply a different class of “slave”.
The same can be said of the Romans, Persians, Egyptians ...
Then came the middle ages and “Regular People” became known as “Serfs” who became the “property” of whomever the Kings gave the property to. Again, “serf” was simply another word for “slave” although the outright buying and selling of people had fallen out of popularity ...for the most part.
The Renaissance was little different ... Nobility remained fully in control of the lives of the “Common Man” ... the “commoners” were still basically slaves.
From about 1200 to the late 1700s some small percentage of “commoners” were granted the status of a “freeman” but it was honestly simply another classification of slave.
Nobles still could kill you or take your stuff for any reason or no reason without any real fear of reprisal.
Another form of Petty Noble was also created, Merchants. They were barely a step above a “Freeman” however and Nobles could ... and did ... find ways to rob them with impunity.

Whenever your life, your well being, your future, your property, the safety of your family, is in the capricious hands of someone of a “Higher Station” ... a Noble ... you are, a Slave ... you can use any other term you choose, but, that is simply a “slave” by another word. It may be that this person had some better classification ... some more “rights” ... but, if those “rights” were simply stripped from you, you never ACTUALLY had “rights”.
So ... in 1775, the ENTIRE WORLD Population was under a “Slave System” where EVERYBODY ... even Nobles, were beholden ... were essentially ... the Property of a King.

The Revolutionary War ... the War between the Colonists and the British “Government” (ie. King) ... was the Greatest and Most Successful Slave Rebellion ... in the ENTIRE HISTORY of the WORLD.

The Creation and Acceptance of the Articles of Confederation ... later the Constitution AND Especially the Bill of Rights ... was the FIRST Time in the ENTIRE HISTORY of the WORLD ... in Over SEVEN THOUSAND YEARS ... that Mankind STOPPED Being one class or another of SLAVE and Woke Up as FREE PEOPLE. The FIRST People to remove entirely the Shackles of Bondage.

In the Mid 1800s THESE People, finally understanding fully what they had managed to do, Rose Up and FOUGHT for the FREEDOM of OTHERS ... Nobody seems to understand that in LESS Than 80 Years a People who THEMSELVES had only recently been “Slaves” saw to it that the Shackles of Bondage were Struck from Another Peoples’ Lives.

Today, there are SEVERAL Groups of “Nouveau Petty Nobles” in OUR Own Government ... in Our Corporate Structure that are struggling to get US to ACCEPT Those Shackles ONCE AGAIN.
TRUE FREEDOM has existed for BARELY 250 YEARS and Many People are WILLING to Trade It In for a Little Bit of POSSIBLE “Security” ...

Well, it should have been the greatest anti-slavery movement in history, and almost was. But sadly the Southern states had become rather dependent on slave labor during the colonial period and refused to give it up after the revolution. In 1784 Jefferson proposed all new states would be free and a gradual emancipation by 1800. But Congress struck this down by one vote - the Representative from New Jersey was ill and couldn't vote.

"I tremble to recall that God is just, and that his justice shall not sleep forever."

Are you on drugs?

What makes you say that? What would make it "greatest" compared to say, Abolitionism in the United Kingdom? Any legit arguments?