By now, most have seen the jarring dash cam video of police officer Jeronimo Yanez shooting Philando Castile as Castile calmly reached for his licence. Just as shocking, a jury acquitted Yanez. The verdict, in the eyes of many, was just one more piece of evidence for how American laws work to protect the powerful and oppress the powerless.

But while the American legal system can be seen as largely constructed to maintain the status quo, it has also served as an agent of change to expand rights — even in the years before the Constitution was ratified. Activists, even slaves, have used the courts to weaponise American ideals and escape oppression.

Before debates about the Constitution began, states grappled with how to adapt the lofty ideals promised by the Declaration of Independence to the reality of slavery. The first was Massachusetts. Its 1780 Constitution marked Revolutionary America’s first attempt to create new legal and political arrangements that gave individual citizens rights in the newly liberated nation.

Yet in Massachusetts, as in every other American colony, the constitutional promise that “all men are born free and equal” did not hold true for African-American slaves. The application of the law exposed the imbalance between the powerful and the powerless, the included and excluded.

Two Massachusetts slaves highlighted this contradiction. Sheffield’s Elizabeth “Bett” Freeman heard the Massachusetts Constitution read aloud, and the next day approached prominent local lawyer Theodore Sedgwick, asking: “I heard that paper read yesterday, that says all men are created equal, and that every man has a right to freedom. I’m not a dumb critter; won’t the law give me my freedom?”

Sedgwick took Freeman’s case.

In May 1781, the same month that Bett’s case was heard in Great Barrington’s County Court, a Worcester slave, Quock Walker, sued his former master, Nathaniel Jennison, for battery. Walker, likewise believing he had a legal right to freedom, had run away from Jennison and gone to work at the neighbouring Caldwell farm, where the abolitionist brothers John and Seth Caldwell helped Walker to find a lawyer and take his case to Worcester County Court.

The county courts decided in both Freeman’s and Walker’s favour. The Massachusetts Supreme Judicial Court, the state’s highest legal authority, was tasked with enforcing the state’s foundational laws and applying its promised rights and freedoms to all residents. Freeman, Walker and their allies pressed the court to decide whether the Constitution’s laws and rights pertained to slaves, hoping to change the conversation to include, rather than exclude, this Massachusetts community.

It worked. The Supreme Court Chief Justice, William Cushing, explained that the 1780 Constitution and the new nation’s ideals rendered slavery illegal because “a different idea” had taken hold when the Constitution declared “all men are born free and equal”. As a result, he could conclude only that slavery was “inconsistent with our own conduct and Constitution”.

Within a decade, pressured by both the court decisions and their communities, Massachusetts slave owners voluntarily freed their slaves, often by changing the arrangements to those of wage labour. The 1790 federal census listed no slaves in Massachusetts, making it the first state comprehensively to abolish slavery.

Abolition in Massachusetts happened because Freeman and Walker took the state’s and the country’s founding laws and precepts at their word. In highlighting the contradiction between concepts of equality and rights, and the circumstances of slavery, they found powerful allies who helped to bring their cases to the state’s most powerful legal bodies, forcing collective decisions that would reverberate across the state.

Protection of the powerful is written into the law of the land, but so too are avenues to use ideas of freedom and equality to change communal conversations and legal practices. And it is this tradition that has begotten constitutional victories expanding rights and freedoms to increasingly greater number of Americans over the past two centuries.

A San Francisco-born Chinese-American cook worked with attorneys and community organisations to win the 1898 Supreme Court case United States v Wong Kim Ark, which made clear that the 14th Amendment’s promise of birthright citizenship should apply to all Americans.

When that promised citizenship was still not extended to Native Americans, Yakama performer Nipo Strongheart and other native activists gathered tens of thousands of signatures on petitions, allied with the Indian Rights Association, and pressured Congress to pass the 1924 Indian Citizenship Act.

And it was individual African-American parents in Topeka pursuing educational opportunities for their children who worked with NAACP lawyers and their allies to win Brown v Board of Education in 1954.

The landmark Supreme Court decision demonstrated that all Americans were included equally in the public education system, began the dismantling of Jim Crow segregation and launched the Civil Rights Movement. Those parents, like Strongheart, Wong, and Freeman and Walker before them, used ideas to create a more just society, providing hints as to how today’s activists can best work to achieve progress.

•Ben Railton, professor of English and American studies at Fitchburg State University, is the author of four books, numerous online articles and a daily blog of public American studies scholarship