President Barack Obama once called him “the most popular politician on earth.” But on July 12, former Brazilian President Luiz Inácio Lula da Silva was sentenced to almost a decade in prison for corruption and money laundering. Known widely as Lula, he is the biggest fish caught so far in a graft probe that has spread across dozens of countries and snared dozens of politicians. The current Brazilian president, Michel Temer, also faces corruption allegations while his predecessor, Dilma Rousseff, was impeached last year under a cloud of suspicion over a massive kickback scheme involving the state-owned oil company Petrobras.

Yet even as the world notes Lula’s downfall, it should also learn why Brazilians have come to demand honesty in their leaders – and in their daily interactions with government. What is perhaps the world’s largest anti-corruption investigation carries lessons for other nations that assume they are trapped in a culture of impunity as Brazil once was.

“In Brazil there is a consciousness about this problem as there never was before,” said federal prosecutor Deltan Dallagnol in an April talk at Harvard Law School, his alma mater. “Society gave us a lot of support.”

The key idea now more widely supported in Brazil is that of equality before the law, even for someone who was once immensely popular and powerful as Lula. “No matter how important you are, no one is above the law,” said Judge Sérgio Moro in handing down his verdict against the former president, who held office from 2003 to 2011.

Brazil restored its democracy only about a quarter century ago, but the system has been flawed by too many political parties relying on too much money for campaigns and in winning votes in Congress. As political scandals have grown, so too has a small cadre of idealistic and well-paid civil servants as well as a popular movement that has steadily pushed legislation and emboldened the justice system.

Yet it is not enough to simply prosecute powerful people, said Mr. Dallagnol. Society, he says, has “provided a shield.”

Justice officials, for example, have created comic books and board games with anti-corruption themes for children. They have also opened up public records about how politicians spend money. For the first time, prosecutors set up a website to expose pending criminal cases. A popular drive to pass anti-corruption proposals drew more than 2 million signatures. And more than 15,000 people in law enforcement took newly designed courses in how to combat corruption and money laundering.

In 2013, as people became aware of overspending for the 2014 World Cup soccer games in Brazil, anti-corruptions protests began to escalate. Also, prosecutors got a big break in a case known as Operation Car Wash, which exposed huge payoffs to politicians by contractors for Petrobras. The new attitude among Brazilians was essential. “We are pretty aware that without public support that this case is not going anywhere,” says Dallagnol. One poll in January found that Brazilians support the investigations of political figures “to the end, regardless of the outcome.”

Many reforms are still needed and prosecutors fear ruling politicians can still thwart their work. “It’s not enough to take out rotten apples from a basket. You need to change the conditions which make those apples to get rotten,” says Dallagnol.

Yet the momentum toward transparency and accountability seems assured – especially when the mightiest of politicians can fall before the public’s heightened demand for equality before the law.