Donald Trump is the only U.S. President elected with no political and/or military experience.

He’s also the first to prompt calls for impeachment so quickly.

“To happen this early is unprecedented,” said University of Houston political science professor Brandon Rottinghaus.

He says many presidents have faced varying degrees of scrutiny.

“As a political argument, it tends to be bandied about, but is almost never in reality something people would turn to,” Rottinghaus said.

In fact, only two presidents have actually been impeached: Andrew Johnson in the 1860s, and Bill Clinton in 1998 amid the Monica Lewinsky scandal.

But both of them were acquitted by the U.S. Senate and never removed from office.

Despite popular belief, Richard Nixon resigned the presidency before he was impeached, although the impeachment process had begun.

Meanwhile, some calls for impeachment may surprise you.

Civil War general Ulysses S. Grant’s corrupt administration led to an effort to oust him. Harry Truman was in the hot seat in 1951 after he fired World War II hero General Douglas MacArthur.

In 1987, Texas Congressman Henry Gonzalez called for Ronald Reagan’s impeachment over the Iran Contra scandal. Years later, Gonzalez introduced a resolution to impeach President George H.W. Bush for starting the Gulf War.

“Impeachment is always on the table, people always talk about it, but it’s almost never legitimately, seriously considered,” Rottinghaus said.

Such an early discussion of impeachment may be indicative of things to come for Trump.

“Even if this doesn’t end in impeachment, there will definitely be a long legacy of investigations in the Trump presidency,” Rottinghaus said.

The founding fathers left rules for impeachment vague in order to adapt to the political and legal circumstances of the times.

Experts say much is left up to interpretation.