Donald Trump has again found himself making headlines for the wrong reasons, this time attracting international condemnation over the treatment of migrant families at the US border with Mexico.

The separation of as many as 2,000 children from their parents and their subsequent incarceration caused uproar around the world when pictures were published this week, forcing the US president to row back on his “zero tolerance” policy.

On Wednesday, he signed an executive order cancelling the practice, which has been branded “inhumane” by the British prime minister, Theresa May.

The New York Times have traced his hard-line stance back to Stephen Miller, a 32-year-old White House senior policy adviser who routinely writes Mr Trump’s speeches.

“The US government has a sacred, solemn, inviolable obligation to enforce the laws of the United States to stop illegal immigration and to secure and protect the borders. There is no straying from that mission,” Mr Miller argued when he was asked if pictures showing children being taken from their parents would make the president change his policy.

“Stephen actually enjoys seeing those pictures at the border,” an unnamed White House colleague told Vanity Fair.

“He’s a twisted guy, the way he was raised and picked on. There’s always been a way he’s gone about this. He’s Waffen-SS.”

Born and raised in Santa Monica, California, his mother’s family emigrated from Belarus in the early 20th century to escape antisemitic pogroms.

Mr Miller nevertheless quickly developed a reputation as a staunch conservative in high school, taking particular inspiration from National Rifle Association CEO Wayne LaPierre’s book Guns, Crime and Freedom (1994).

He first attracted attention in 2001 when, aged 16, he wrote a letter to the editor of The Santa Monica Outlook in the wake of 9/11 suggesting “Osama bin Laden would feel very welcome at Santa Monica High School” because of his fellow students’ opposition to George W Bush’s War on Terror.

He became a frequent contributor to right-wing talk radio in the Los Angeles area and was known on campus for his contrarian attitudes, telling Hispanic students not to speak Spanish and arguing during a student government speech he and his peers should not have to pick up their own litter as “[there are] plenty of janitors who are paid to do it.”

Going on to attend Duke University in Durham, North Carolina, to study political science, his reputation as a Republican influencer grew and he achieved new prominence when he appeared on Fox News’s The O’Reilly Factor in 2006 to defend members of the institution’s lacrosse team who had been accused of rape.

At Duke, Mr Miller met Richard Spencer, a white nationalist who claims he mentored the future Trump staffer, a version of events Mr Miller disputes.

Graduating in 2007, Mr Miller went to work as a spokesman for the Republican representatives Michele Bachmann and John Shadegg in Minnesota and Arizona respectively.

Two years later, he served as a policy adviser and communications director for Jeff Sessions, now Mr Trump’s attorney-general.

Together, the pair opposed a bipartisan bill in 2013 which would have created a path to US citizenship for illegal immigrants.

During his tenure with Mr Sessions, Stephen Miller developed the term “nation-state populism” and befriended Fox anchor Tucker Carlson, a key media ally.

Mr Miller joined the Trump campaign team in 2016, initially as a senior adviser before specialising in economic policy, often speaking at rallies in addition to writing for the presidential candidate.

He notably composed Mr Trump’s July 2016 address to the Republican National Convention, inaugural address and first State of the Union speech.

He became fast friends with the president’s former chief strategist, Steve Bannon, and retains ties to the latter’s alt-right publication Breitbart News.

Opposition to immigration has remained the defining issue of Mr Miller’s hawkish career so far – in spite of his personal family history - and led to several heated encounters with reporters like CNN’s Jim Acosta and Jake Tapper in defence of the president’s “Muslim travel ban,” which bars entrance to the US for citizens from seven predominantly Muslim countries.

Undocumented immigration across the US-Mexico border Show all 14 1 /14 Undocumented immigration across the US-Mexico border Undocumented immigration across the US-Mexico border Immigrant children, many of whom are separated form their parents, are housed in Texas' tent city Reuters Undocumented immigration across the US-Mexico border A two-year-old Honduran asylum seeker cries as her mother is searched and detained near the US-Mexico border Getty Undocumented immigration across the US-Mexico border Undocumented migrants ride on the top of a freight train referred to as the beast, or La Bestia Getty Undocumented immigration across the US-Mexico border A cage inside a US Customs and Border Protection detention facility in Texas Reuters Undocumented immigration across the US-Mexico border US Border Patrol Academy All new agents must complete a months-long training course at the New Mexico facility before assuming their posts at Border Patrol stations, mostly along the US-Mexico border Getty Undocumented immigration across the US-Mexico border US-Mexico border fence A group of young men walk along the Mexican side of the US-Mexico border fence in a remote area of the Sonoran Desert Getty Undocumented immigration across the US-Mexico border US-Mexico border fence in the US Man looks through US-Mexico border fence into the US in Tijuana, Mexico Getty Undocumented immigration across the US-Mexico border US-Mexico border fence US Border Patrol agent Sal De Leon stands near a section of the US-Mexico border fence while stopping on patrol on in La Joya, Texas Getty Undocumented immigration across the US-Mexico border US Border Patrol Academy US Border Patrol instructor yells at trainees after their initial arrival to the academy Getty Undocumented immigration across the US-Mexico border Memorial service in Guatemala Families attend a memorial service for two boys who were kidnapped and killed in San Juan Sacatepequez, Guatemala. Crime drives emigration from Guatemala to the United States, as families seek refuge from the danger Getty Undocumented immigration across the US-Mexico border Arrests on the border Undocumented immigrants comfort each other after being caught by Border Patrol agents near the US-Mexico border Getty Undocumented immigration across the US-Mexico border Detention holding facility A boy from Honduras watches a movie at a detention facility run by the US Border Patrol Getty Undocumented immigration across the US-Mexico border Mexican farm workers Mexican migrant workers harvest organic parsley at Grant Family Farms in Wellington, Colorado Getty Undocumented immigration across the US-Mexico border Mexican family in Arizona A Mexican immigrant family sits in the living room of their rented home in Tuscon, Arizona. The family that Arizona's new tough immigrant law had created a climate of fear in the immigrant community. Getty

“As long as Stephen Miller is in charge of negotiating immigration, we are going nowhere,” senator Lindsey Graham said during the government shutdown negotiations in January.

Apparently partial to Mexican food all the same, Mr Miller was branded a “fascist” by a fellow diner at the Espita Mezcaleria restaurant in Shaw, Washington, DC, on Sunday.