(CNN) The House has launched a formal impeachment inquiry into President Donald Trump. While many US presidents have been threatened with impeachment, Congress has only conducted two presidential impeachment trials.

How impeachment works

A sitting US president can be impeached for treason, bribery or "other high crimes and misdemeanors." The House of Representatives votes for impeachment, and if a majority of members vote in favor, the Senate conducts a trial. A two-thirds majority in the Senate is required to convict and remove a president from office — which has never successfully happened.

Richard Nixon wasn't actually impeached

From the start of the Watergate investigation, Nixon was adamant about his innocence. He repeatedly stated that he never told any member of his staff to do anything illegal and that he knew nothing about the break-in at the Democratic National Committee headquarters during the 1972 presidential campaign that sparked the investigation.

In an effort to clear his name, Nixon stressed his willingness to cooperate with investigators. But as the probe dragged on, Nixon unsuccessfully tried to block prosecutors' demands for evidence. This resulted in the landmark US v. Nixon decision in July 1974, where the Supreme Court unanimously ordered Nixon to turn over damaging White House tapes to investigators.

Nixon ended up facing possible impeachment for obstruction of justice, abuse of power, and contempt of Congress during the Watergate scandal. He resigned on August 9, 1974, before the House could vote.

But Andrew Johnson was ...

President Andrew Johnson

Congress has only conducted two presidential impeachment trials and both presidents were acquitted and stayed in office.

The first was in 1868, involving President Andrew Johnson for firing a cabinet secretary without the consent of Congress.

When Johnson tried to force out Secretary of War Edwin M. Stanton in 1867 for conspiring with his political enemies, radical Republicans began moving for impeachment.

Their first attempt, in December 1867, failed, but on February 24, 1868, they succeeded. The House passed a resolution of impeachment and followed that up by issuing 11 articles detailing the charges against the president. Foremost among them was the largely political charge that Johnson had violated the recently-passed Tenure of Service Act in the Stanton affair.

The Senate trial opened on March 5, with Chief Justice Salmon P. Chase presiding. It lasted three weeks. Johnson never appeared. In the end, the Senate court came close to convicting him on three of the charges, but in each case fell one vote short of the required two-thirds majority.

... and so was Bill Clinton

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President Bill Clinton was impeached in 1998 on perjury and obstruction of justice. The genesis of the case against Clinton started in the Whitewater affair, and the settlement of a sexual harassment lawsuit brought by Paula Jones.

The litigation sparked an investigation into whether Clinton obstructed justice and lied under oath. The probe centered on Clinton's relationship with a White House intern, Monica Lewinsky.

After a four-year investigation, special prosecutor Kenneth Starr produced a 445-page report detailing Clinton's relationship with Lewinsky. The report lists acts that could be grounds for impeachment. On October 8, 1998, the House approved the impeachment inquiry.

Four articles of impeachment were set forth in the House. Two articles were approved: One alleges that Clinton committed perjury when he told a grand jury that he did not have an affair with Lewinsky and the other alleged that Clinton coerced Lewinsky to lie under oath about their relationship. In January 1999, an impeachment trial began in the Senate, but Clinton was acquitted a month later.

The impeachment threat comes up a lot: Every president since Ronald Reagan has been threatened with impeachment by members of the House, including Trump, who faced three separate resolutions for impeachment in 2017.