Donald Trump's choice of far-right publishing executive Steve Bannon as a top White House adviser is bringing new scrutiny to a troubling, decades-old ideology: white nationalism.

The movement generally advocates formation of a nation set aside for whites. Some adherents openly supported Trump for president, and white nationalists have praised Trump's appointment of Bannon as a senior adviser.

Bannon previously headed the Breitbart website that appealed to the so-called 'alt-right' a movement often associated with far right efforts to preserve 'white identity,' oppose multiculturalism and defend 'Western values'.

White nationalists often support the idea that white people are under attack in the US, and need protection from the growth of minority and immigrant groups.

Adherents sometimes use the hashtag #whitegenocide on social media to promote their belief that the future of the white race is in peril. They see diversity as a threat to fight, not a goal to embrace.

Here are some questions and answers about white nationalism in the United States:

How did this get started?

Groups including the Ku Klux Klan, which is 150 years old, have espoused various white nationalist ideas.

The start of the current white nationalist movement is pegged to more recent years.

JM Berger, an author and expert on extremism at George Washington University's Program on Extremism, wrote earlier this year that many of today's white nationalists were inspired by 'The Turner Diaries', a racist novel published in 1978.

In the book, physicist-turned-writer William Luther Pierce describes a dystopian America in which white people are disarmed by minorities.

Timothy McVeigh had pages from the book with him when he bombed the Alfred P. Murrah federal building in Oklahoma City in 1995, killing 168 people.

Is Steve Bannon a white nationalist?

Bannon hasn't commented publicly since being tapped for the White House position, but his former colleagues at breitbart.com dispute any links to white nationalism.

In a statement released to The Hill, a political newspaper published in the nation's capital, breitbart.com said it had been under intense scrutiny over Bannon's involvement with Trump and was preparing a lawsuit over media depictions of it as a “white nationalist website.”

The statement added: “Breitbart News rejects racism in all its varied and ugly forms. Always has, always will.”

Speaking on CNN, a Trump spokesman has called media coverage of Bannon's appointment 'irresponsible'.

Are white nationalists organised?

To an extent, yes.

One such group, the National Alliance, was actually founded by Pierce, the Turner Diaries author.

As with any such group, accurate membership numbers are tough to attain. And the National Alliance's website says it won't accept just anyone into the fold: Only heterosexual whites who aren't addicted to alcohol and drugs are welcome.

Members can't be involved romantically with a person of another race, and they can't be in prison.

A post-election commentary posted on the National Alliance's website called Trump's victory “a move in the right direction” and “a temporary reprieve for the United States of America as a majority-white country.”

Another group with white nationalist leanings, the Council of Conservative Citizens, also has praised Trump's election, as have various KKK groups and the American Nazi Party.

White nationalist groups embrace a number of different symbols. Some with Nazi roots use the swastika as an insignia; KKK groups with white nationalist beliefs use more traditional Klan insignia, like a cross with a red tear. The National Alliance website features a cross with arms bent upward.

Isn't all this just prejudice with a new name?

It depends on who you ask.Critics of white nationalism definitely see it as a racist ideology, a form of hate run amok.

They say the very idea of establishing a “white” nation is disparaging and hurtful to the racial minorities, ethnic groups and religious adherents who would presumably be forced to live elsewhere.

Believers in white nationalism disagree. They deny looking down upon other races or believing they are superior to anyone.

They say they don't hate. They just want their own country, which would implicitly allow minorities to have their own nation.

Are there other kinds of nationalists?

Absolutely, including black nationalists.

One of the founders of black nationalism was Marcus Garvey, who founded the Universal Negro Improvement Association in 1914.

The Nation of Islam, formed in Detroit in the 1930s and now led by Louis Farrakhan, also has a black nationalist strain, as the religion aims in part to free blacks from “servitude” to Western civilization white society.

Other black nationalist groups have popped up through the years, and several have an internet presence. The gunman in the Dallas police massacre, Micah Johnson, had shown interest in black nationalist groups before opening fire and killing five officers earlier this year.

The Alabama-based Southern Poverty Law Center, a liberal advocacy group which monitors extremist organisations, classifies both white nationalist and black separatist organisations as racist.