Final answer:
The decision that mandated states must provide legal defense for individuals who cannot afford it is Gideon v. Wainwright, a significant Supreme Court case establishing the right to an appointed attorney.
Step-by-step explanation:
The legal decision that required states to provide for the legal defense of indigents is known as Gideon v. Wainwright, a landmark Supreme Court case. In 1963, the Supreme Court heard the case of Clarence Earl Gideon, a Florida man who could not afford an attorney and was denied a public defender by the state for his felony charge. The Court unanimously decided that the Sixth Amendment's guarantee of counsel was a fundamental right essential for a fair trial, and thus it was obligatory for states to provide attorneys for defendants who could not afford one.
Notably, the Gideon v. Wainwright ruling overturned the previous decision in Betts v. Brady, which had allowed states to deny counsel to indigent defendants under certain circumstances. The Gideon decision is a cornerstone in criminal law, ensuring that the right to legal representation is not dependent on the accused's financial status, thus reinforcing the principle that the burden of proof lies with the prosecution and defendants are given the benefit of the doubt until proven guilty.