One of the key obstacles to the release of the last British prisoners-of-war in Moscow was the ongoing conflict between Poland and Russia in 1920.
Poland had achieved independence after World War I, but began a war with the Bolsheviks to regain territory lost to Russia one hundred years before. Fighting continued for 18 months before General Pilsudski defeated the Red Army at Warsaw and followed this up with a successful advance east towards Moscow.
Lenin sued for peace and the two sides agreed a cease-fire in Riga on 12th October that went into effect a week later.
There were still many questions to answer. What was to happen to Ukraine and Belarus, who fought with Poland for their independence? Would the Allies accept the new frontier which was 150 miles east of the Curzon Line that had been agreed at the Paris Peace Conference? The Peace of Riga was eventually signed five months later, but sadly it did not resolve the key issues, which festered until World War II.
However, the cease-fire on 18th October did solve one problem and that was the freedom of Captain Brian Horrocks and the other British prisoners held in Moscow.