Stephen Miller (Nicholas Kamm/AFP/Getty)

Acting Homeland Security Director Kevin McAleenan blocks the latest attempt by Donald Trump’s senior advisor Stephen Miller to extend his control of anti-immigrant policies and operations.

McAleenan’s showdown with Miller came over Mark Morgan, a former FBI official designated by Trump as the new director of Immigration and Customs Enforcement.

Miller intervened to keep Morgan out of ICE, having him installed as the new commissioner of Customs and Border Protection.

But McAleenan said he might quit unless he was given more authority with Homeland Security and related agencies. He told senior White House officials that he — not Miller — was in charge of DHS, that he should make personnel decisions or at least be involved in them, and that communications must improve.

A Trump aide said the episode was an “immigration knife fight”, with McAleenan meeting White House Chief of Staff Mick Mulvaney.

McAleenan succeeded for now, with Morgan confirmed as acting ICE director next week. Matthew Albence, the current acting director and Miller’s preference, will return to his job as ICE deputy director.

Miller’s Power Play

Miller, the 33-year-old who only four years ago was best known for spamming journalists and legislators with his ideas, has become the driving force in Trump’s “zero tolerance” approach to immigration.

The advisor spurred policies such as the separation of children from undocumented migrant parents, the expansion of detention, and the declaration of a “national emergency” for Trump’s Wall with Mexico.

Miller is also seeking to deport some legal immigrants from the US, if they have received public assistance within five years of arrival.

He has bolstered his authority by encouraging Trump to remove other officials, saying they are weak or trying to undercut the White House. Last month Trump sudden withdrew the nomination of Ronald Vitiello to become ICE director. Three days later, Homeland Security Director Kirstjen Nielsen was dismissed.

Before their departure, Nielsen and Vitiello challenged a plan devised by Miller and Albence to carry out mass arrests of migrant parents and children in 10 US cities.