The Man from St Petersburg book by Ken Follett

His name was Feliks.  He came to London to commit a murder that would change history.  A master manipulator, he had many weapons at his command, but against him were ranged the whole of the English police, a brilliant and powerful lord, and the young Winston Churchill himself.  These odds would have stopped any man in the world-except the man from St. Petersburg...

The Man From St Petersburg is a dark tale of family secrets and political consequences. Ken Follett's masterful storytelling brings to life the danger of a world on the brink of war. A Secret Negotiation 1914. Tensions are rising as Europe finds itself caught in a web of alliances and dangerous warmongering. To help tip the balance in their favour, Britain aims to draw Russia into an alliance with them instead of Germany. Czar Nicholas’s nephew, Prince Aleksei, is sent to London for secret naval talks with Lord Walden. A Play for Power Walden has a personal connection to Aleksei; his wife is Aleksei’s aunt. And they are not the only ones interested in the Russian's arrival; so too are Walden’s willful, idealistic daughter, the head of Special Branch, and a ruthless Russian anarchist. A World at War With the British desperately needing a signed treaty and the fate of the world hanging in the balance, the destinies of these individuals become inextricably linked, as tragedy threatens to shatter the Waldens’ complacency . . .