Trump wave
© Gage Skidmore/flickr, CC BY-SA
Beware Trojan horses.
During New Year celebrations in Cologne, there were more than 500 reported attacks against women, including robbery and sexual assault. Most of the suspects are of North African origin, and some are thought to have entered the country illegally or as asylum seekers.

The news was welcome campaign fodder for US presidential hopeful Donald Trump. Referring to German chancellor Angela Merkel's open door policy on refugees from Syria, he commented in his usual rhetoric: "I don't know what the hell she is thinking".

Trump went on to say that he did not want to have "people coming in from migration from Syria (sic)" as these were aggressive young men who "look like they should be on the wrestling team". More dangerously still, Trump believed such people could act as terrorist "Trojan horses".

Trump's comments are in line with his vicious verbal attacks on Mexicans and other immigrant groups in the United States. But they betray his own family background. His grandfather, Friedrich Trump, a German, lived a migrant life in the US on the edge of illegality and rejection. During the World War I, he belonged to an immigrant group which was sweepingly labelled the "enemy within" or - in his grandson's parlance - a Trojan horse.

The great wave

Friedrich Trump was swept to the United States in one of the biggest waves of mass migration in history. During the 1880s and early 1890s, 1.8m Germans emigrated to various European and overseas destinations. When young Friedrich arrived in New York in 1885 he joined around 200,000 of his compatriots who had already settled in the metropolis, forming a distinct "Little Germany". After working for six years as a barber, he was caught by the Gold Rush, moved west and opened up a chain of restaurants and hotels in Washington State and British Columbia. Hospitality did not only include food and lodging, but also alcohol and prostitution. Friedrich anglicized his name to Frederick and became a US citizen.

By 1901, Frederick had made a small fortune and decided to return to his hometown of Kallstadt in south-west Germany. (Incidentally, the Heinz family of Ketchup fame has its origins in the same town.) Frederick married his childhood sweetheart, Elisabeth, and planned to settle down.

Trump grandfather

Frederick Trump
The Bavarian Palatinate authorities, however, would not let him. They claimed he had left Germany as an illegal emigrant, evading taxes and the compulsory two-year military service. Frederick pleaded that he and Elisabeth were "loyal Germans and stand behind the high Kaiser and the mighty German Reich". It was all to no avail. The Trumps were evicted and resettled to New York.

Wartime spy fever

World War I was not a happy time for German-Americans. They were summarily labelled as "alien enemies" whose true allegiance lay with the Fatherland. Nativist spokesmen agitated against "hyphenated Americans" as potential spies and saboteurs. Use of the German language was seen with suspicion. In contrast to many of their compatriots, the Trumps did not need to anglicize their surname as it worked perfectly in English.

The most notorious case of public violence was the lynching of German immigrant Robert Prager in Illinois. He was tarred and feathered, forced by an agitated crowd to kiss the American flag and sing patriotic songs, and finally hanged from a tree in front of 200 onlookers.

Frederick Trump evaded the fate of Prager, but not the other deadly weapon which swept the world once the war was nearing its end. In 1918 and 1919, Spanish influenza killed between 20m and 50m people worldwide. On a summer's day in 1918, Frederick returned home from a stroll through New York with his son Fred (Donald's father), went to bed feeling sick, and passed away the next day.

Paranoid nation

The dangerous mix of paranoia and xenophobia directed against German-Americans during World War I had profound and long-lasting effects. The Alien Enemy Bureau was established in the early days of the war with a brief to identify and arrest disloyal foreigners. It was headed by J. Edgar Hoover, then a young civil servant in the Justice Department. Here he picked up the tools he would use later as all-powerful director of the FBI.

In 1940, the notorious House Un-American Affairs Committee published The Trojan Horse in America, a compendium of domestic organizations believed to work for foreign powers. Chapter titles included "Mussolini's Trojan Horse in America" and "A Trojan Horse of German War Veterans".

All this was reason enough for the business-minded Trumps to deny their German heritage, claiming they hailed from Sweden instead. Donald's father Fred invested heavily in New York real estate, laying the foundations for today's business empire. It was only from the 1980s that Donald Trump started to stand by his German roots.

Trump's own grandfather was an illegal emigrant whose income stream included alcohol and prostitution at a time when these were legally contested. He was an unwanted returnee to Germany, and then a potential "enemy alien" within the United States who had declared his loyalty to the German Kaiser - but ultimately made an immense economic contribution spanning generations.

Today, his grandson lambastes Mexicans as criminals, intends to erect a wall to keep them out, and warns of Syrian refugees as Trojan horses. If Donald Trump wins his party's nomination, historians will have many a field day digging out the contradictions between his anti-immigrant rhetoric and his family background.