White nationalists believe that there should be territories in the United States that are just for white people, and other territories just for blacks, and others just for Native Americans, and that Hispanics should go back south of the border (or maybe some who have been here a long time should have their own territory inside the United States.) Some white nationalists say they are opposed to violence to achieve their goals; they say violence is counterproductive (it produces a backlash) and they say it is not necessary because eventually white nationalists will persuade most people to agree with them. Of course some white nationalists murder people just because they are not white, as recent mass shootings demonstrate.
White nationalists believe there needs to be separation of the races because people of a given race innately (it's a genetic, biological thing) want to live with their own kind and there will therefore inevitably be friction and regrettable discord when they live together.
Some white nationalists also believe that it is important to keep whites separate from non-whites in order to prevent race mixing that would dilute the purity of the white gene pool.
Here's where these white nationalists are wrong:
1. Enforcing racial segregation only benefits an oppressive ruling elite; it is a way of destroying solidarity between the working class people of the different races. This is exactly why the upper class in the American South instituted Jim Crow segregation.  Forced racial or ethnic separation, such as apartheid in South Africa and Israel's ethnic cleansing to ensure that the population of the 78% of Palestine now known as Israel will always be at least 80% Jewish, are methods by which the billionaire ruling elites pitted people against each other in order to dominate and oppress and exploit ALL races and ethnic groups of working class people. [1.b]
2. The supposed value and importance of maintaining the purity of the white race's gene pool is entirely based on crackpot science. 
3. White nationalists point to the friction and discord between (mainly white and mainly working class) American citizens and Hispanic immigrants (especially the illegal ones) as proof that "the races just cannot mix peacefully." But the white nationalists fail to realize that this friction and discord doesn't show that different races cannot mix peacefully; what it shows is that when the American upper class creates a living hell south of the border to FORCE *massive* numbers of people to illegally immigrate into the United States in order to survive, and when the upper class uses these people as cheap labor (cowing them into submission with the threat of deportation) that drives many American workers out of what used to be higher paying jobs, and when the obscenely rich upper hogging class deprives local communities of the wealth they should and would have in a more equal society and then tells these communities they have to provide for the various needs of this HUGE influx of immigrants, then yes, friction and discord result, duh. The friction and discord would have resulted even if the immigrants were racially the same (whatever the hell that might mean) as the white American citizens. 
4. White nationalists fail to realize that the most important thing about people is not their race (or their genes), but their values. The vast majority of people value no-rich-and-no-poor equality and mutual aid (I call them egalitarians whether they've even heard that word or not), while a small minority value inequality and domination of the many by the few (the proper academic term for such persons is assholes.) Egalitarians--the vast majority of people--have FAR more in common with each other than they do with assholes, totally regardless of "race." If there should be any exclusive territories to prevent serious and regrettable discord and friction inside them, then there should be territories exclusively for egalitarians of any and all races, and from which assholes of any and all races would be excluded.