The 1963 Supreme Court case Gideon v. Wainwright enforced the right for legal counsel during a trial and helped form the basis of our legal system today.
Sources
https://www.landmarkcases.org/cases/gideon-v-wainwright
https://www.floridasupremecourt.org/Law-Library/Historical-Documents-Rare-Books/Gideon-v.-Wainwright
https://www.oyez.org/cases/1962/155