Two prosecutors working Robert Mueller’s investigation into Russian interference in the 2016 election have left and returned to jobs at the Justice Department, a possible sign that the investigation is winding down. Among the big questions remaining, after the indictments of Trump campaign staff or confidants: Will Mueller formally charge President Donald Trump with a crime?

As media coverage has underscored, there is a long-standing tradition of American jurisprudence that a president cannot be indicted while in office.

Ancient roots

Immunity from prosecution for elected officials is not a modern American concept. It stretches back almost three millennia to the ancient Mediterranean.

Paying attention to this history, and particularly to the tumultuous last days of the Roman Republic in the first century B.C., allows us to better understand how our current conversation about presidential immunity is deeply entwined with ancient Roman law, whether we know it or not.

As classics scholar Sarah Bond has recently shown, immunity “either as a privilege of office or as a special grant” has a long history as a feature of Greek, Roman and medieval law.

Immunity in Roman Law

While both Roman and U.S. law render certain officeholders immune from prosecution while in office, the Romans theorized the relationship between power and office differently than U.S. law.

Roman law granted immunity to certain elected officials whose offices entitled the holder to “imperium” or “potestas” or to officials whose office was “sacred.”

Offices with “imperium” are closest to what we might consider presidential powers. The term translates generally as “the right to command.” “Imperium” allowed the officeholder to exercise authority over a range of matters, such as military command, legislative authority, the maintenance of public order and the power of coercion (ranging from incarceration to capital punishment).

Protection has its limits

According to Roman jurists, officials with immunity could not be summoned to court for criminal or civil offenses while they held office.

This did not mean, however, that those officials could never be held accountable.

Roman officeholders with immunity could not be removed from office until their term was completed. They kept their office until they formally laid it down in a ritual ceremony. All officeholders could be indicted after they completed their year in office for both civil and criminal offenses.

Since the law stipulated that officeholders with immunity could not hold two such offices consecutively, officials were often taken to court after their terms expired.

One of the most famous examples was the trial of Gaius Verres for his alleged crimes while governor of Sicily in 70 B.C. Verres was brought to trial after his governorship for extorting bribes and looting art from temples, among other things.

As Rome conquered more territory, the temptation for provincial governors to engage in graft or other crimes increased. That’s because new territory presented greater access to resources, while the governor’s “imperium” made taking advantage of the inhabitants of these provinces far too tempting. This meant that accountability became more important over time.

Whereas the American system allows for the possibility of impeaching certain elected officials, the Roman system relied on veto power from a higher official or one of equal rank, and short terms of office.

Thus, a consul – the highest elected office in the Roman Republic and closest in practice to our president – could have his decisions vetoed by his co-consul for that year, effectively rendering his powers null. Because these offices were held for only a single year, this system made it hard for one individual to do lasting damage.

Julius Caesar works the system

The Roman political system began to fall apart in the Late Republic of the first century B.C. This was a time when, much like our current moment, there was a breakdown of norms and the exploitation of the system’s rules by increasingly brazen elites.

The most famous example of this was Julius Caesar.

In 59 BCE, Caesar was first elected consul. Caesar and his opponents used bribery extensively to buy votes during the election.

In order to pass legislation, Caesar also violently intimidated his conservative co-Consul Bibulus to the point where he holed up in his house for the year.

At the end of his term, Caesar arranged to be appointed proconsul, or governor, of Gaul for five years, the result of a secret alliance with Marcus Crassus, a wealthy aristocrat, and Pompey, a powerful military commander. This extended his immunity and allowed him to avoid pending prosecution.

The First Triumvirate – as these three were called – later extended Caesar’s proconsulship by another five years.

By 50 B.C., having held office for 10 years, Caesar’s alliance had fallen apart and he lacked the allies in Rome to secure a further renewal of his term. When his term was set to end, he feared prosecution for what he had done during his consulship.

The next year, Caesar led an army across the Rubicon in a bid to hold on to his office, beginning a civil war that would lead to the fall of the Republic.

All because he was afraid of losing his immunity.

US impeachment vs. Roman veto

While British law was informed by Roman law, the founders of the American Republic relied heavily on Roman legal philosophy as a model when they wrote the Constitution.

In the Federalist Papers, Alexander Hamilton described presidential immunity much like that which applied to a Roman consul: The president cannot be prosecuted while in office, but he is liable to prosecution after his term ends.

Unlike the Roman system, Hamilton and the founders created a mechanism to remove the president from office: impeachment by Congress. Impeachment was a necessary check. With two consuls serving alongside each other every year, the Romans had a natural check on executive power that is lacking in the American system and its single executive.

This brings us back to Trump’s legal troubles. Roman law relied on short terms of office and the veto power of other officeholders as checks on the grant of immunity. The American system relies on impeachment by Congress as the mechanism for checking presidential immunity.

Caesar’s various machinations – extending his time in government, waging war – to prolong his immunity should make it clear what any president, including Trump, could do to maintain their immunity to prosecution. These dangers apply to Republican and Democratic politicians alike.

As was the case with Caesar, immunity is a tough thing to give up when you may have something to lose.

Cavan W. Concannon does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.