Past impeachments, products of their times, often produced unintended consequences

Julian Zelizer : Impeachment of the president has reared its head five times in U.S. history.

He says each episode had a profound impact on direction of national politics, both in the short term and for years to come.

Zelizer says it's not clear whether the "impeachment inquiry" announced last week by Speaker Kevin McCarthy will even lead to formal charges or a vote in the House .

The last president impeached was Bill Clinton , who was in his second term when an independent counsel investigating other matters found evidence that Clinton had a sexual affair with a 22-year-old White House intern.

Clinton was widely pilloried for his behavior, and lying about it, but the impeachment itself was far from popular.

Nixon was only the second president actually impeached, because the Nixon process was short-circuited in 1974 .

Andrew Johnson was impeached in 1868 after the assassination of Abraham Lincoln and the Civil War .

John Sutter : The only other example of impeachment that U.S. history offers is a distant one, but it provides a further illustration of how political circumstances can be as important as a president's actual misdeeds.

Johnson was enmeshed in a war with the hardcore Northern senators who were zealous about freeing the formerly enslaved.