When Britain was compelled to introduce conscription in 1916, the question arose of what to do with its 'friendly aliens'-its 30,000 Russian-Jewish refugees of military age. The Tsar didn't want them back to serve in his army, and they had no desire to help his war effort. But when sections of the British press commented that as asylum seekers they should show gratitude and join up, a campaign with strong anti-Semitic overtones took off and became Parliamentary business. Then the Tsar was overthrown, and by the summer of 1917 the question was settled with the new regime: Russian Jews of military age had to choose either to join the British Army or to return to Russia to serve there. MI5 and Special Branch kept watch on the Communist Club in the West End, where Russian revolutionaries agitated tailors, cobblers and cabinet-makers who agonised over what to do. Many ended up in the British Army or were exempted for war work, but nearly 4,000 chose to go back to Russia-for a variety of reasons. Arriving there via the Arctic within weeks and days of the Bolshevik revolution, they found a vacuum of authority, which soon turned into the chaos and dangers of civil war. How they fared and how they struggled to return to their families in Britain is the story of War or Revolution. With the help of personal documents, official British and Russian archives, and interviews of survivors and their families, the author illuminates an unknown corner of the First World War and sets it against the background of the period.