Sid­ney Reil­ly was a spy — a mas­ter spy, as the sub­ti­tle of this biog­ra­phy describes him. But he was also an enig­mat­ic fig­ure whose espi­onage exploits are in large part con­jec­ture. Phras­es such as ​“it is unclear,” ​“it is like­ly,” and ​“it may have been” accom­pa­ny the account of the man and his activities.

Born in 1874 (or per­haps 1872), Reil­ly pre­sent­ed him­self as Irish-born, a great British patri­ot, and firm­ly anti-Bol­she­vik. The lat­ter descrip­tions were proven true; but Irish-born? No. And his name wasn’t Sid­ney Reilly.

Sig­mund, or Shlo­mo, Rosen­blum may have been born in Odessa (or pos­si­bly in Russ­ian-ruled Poland) to Jew­ish par­ents (although his moth­er may have con­vert­ed to Catholi­cism), where he spent at least part of his youth. He left as a young man, with sev­er­al years unac­count­ed for, mak­ing his way to Lon­don in 1895.

From there, he would become a ​“busi­ness­man, con man, and spy” with an exceed­ing­ly active love life. He changed his name along the way, mak­ing it eas­i­er for him to trav­el and work, and dis­tanc­ing him from his Jew­ish roots. On that, it seems, he nev­er looked back.

Much of the nar­ra­tive of Reilly’s life inter­sects with the polit­i­cal machi­na­tions of the ear­ly twen­ti­eth cen­tu­ry — Tsarist Rus­sia, the Russ­ian Rev­o­lu­tion, oil explo­ration and licens­ing — and, lat­er, the First World War. Spy­ing most­ly for Britain, he prof­it­ed from the war, bro­ker­ing arms deals and doing busi­ness in Britain, Europe, Rus­sia, Japan, and the US, where one of his com­peti­tors was the J. Pier­pont Mor­gan Bank.

Nei­ther Reilly’s ​“deals” nor his espi­onage were straight­for­ward, and he fre­quent­ly came under sus­pi­cion. Some were con­vinced he was a Ger­man agent who, to make mon­ey, was sup­ply­ing arms to Ger­many. There may have been ​“a very small ele­ment of truth” in this, Israeli his­to­ri­an Ben­ny Mor­ris writes. It was, how­ev­er, nev­er proven.

In 1918, Reil­ly applied for a posi­tion with MI6, the British mil­i­tary intel­li­gence; and, despite sus­pi­cions about him, he was hired. While he would work to over­throw Russia’s Bol­she­vik gov­ern­ment to the end of his life, MI6 sev­ered his employ­ment in 1920. Anti­semitism, Mor­ris main­tains, played a part in his dismissal.

In 1925, Reil­ly was lured back to Rus­sia, where he was impris­oned and inter­ro­gat­ed. He was exe­cut­ed sev­er­al months later.

Mor­ris sum­ma­rizes Reil­ly as ​“some­thing of a chameleon, con­stant­ly chang­ing iden­ti­ties, activ­i­ties, and venues,” a rev­o­lu­tion­ary who came to see him­self as ​“the man who would orga­nize and per­haps even lead regime change” in Rus­sia. He might have failed in that, but he suc­ceed­ed in secur­ing ​“a notable place in the annals of espionage.”