During his service in the Auxiliary Division, Nathan was suspected of being involved in the assassination of two Sinn Féin politicians, which later contributed to the alienation of Irish volunteers in the International Brigades from their British counterparts during the Spanish Civil War.

[6] He rose from private to company sergeant major and "after three years and 334 days in the service, he was commissioned in the field on 9 April 1917"[7] to become "the only Jewish officer in the Brigade of Guards".

On October of that year, Nathan joined the Auxiliary Division of the Royal Irish Constabulary (RIC) and was posted to G Company as a Section Leader, being stationed at the Lakeside Hotel in Killaloe, County Clare.

[9] During his service in the Auxiliary Division, Nathan was suspected of being involved with a series of assassinations in Limerick which took place on 7 March 1921, when the sitting mayor of Limerick, Sinn Féin politician George Clancy, councillor and former mayor Michael O'Callaghan and city clerk Joseph O'Donoghue were all shot and killed in their homes.

[10][1] An article published in the New Statesman by Richard Bennett in 1961 stated that two former Auxiliary Division members had anonymously identified Nathan as the killer of the two mayors.

Frank Percy Crozier, a former Auxiliary Division officer, wrote in his book Ireland for Ever that he agreed with Kathleen O'Callaghan (wife of one of the men killed, Michael O'Callaghan) that the mayors were "murdered by police, acting under orders, as part of a plan to 'do away with' Sinn Fein leaders, and put the blame on Sinn Fein".

[11] After returning from Dublin, Nathan relinquished his commission and rejoined the British Army; this time the West Yorkshire Regiment as a private, but had left the military by October 1922 at his own request.

[12] He traveled to Halifax, Nova Scotia in February 1928, intending to become a farmer, but only found work as a salesman, staying in Canada for some years.

Nathan elected to travel to Spain in December 1936, where he joined the mostly French Marseillaise Battalion of the XIV International Brigade, as a Captain of the British Company with it.

[13] Irish volunteers from the Republican Congress also found out that they were subordinated to two former members of the Auxiliary Division from the Anglo-Irish War; Nathan and Wilfred Macartney.

Nathan finally came unstuck at the Battle of Brunete where he was killed as a result of bomb fragments from a Junkers Ju 88 aerial bombardment and died on the 17 July 1937.

At nightfall he was buried in a rough coffin beneath the olive trees near the River Guadarrama ... "Gal" and Jock Cunningham, two tough men who had been jealous of Nathan, stood listening [to the funeral oration] with tears running down their cheeks.