Conscripted into the First Lancashire Regiment in 1943, he served in France, Belgium and Germany and he was also involved in the mutiny in Egypt after the end of the war, earning him three months in military prison.
[2] When factional disputes broke out in The Club (the name adopted by British Trotskyists after entering the Labour Party) Hallas became a supporter of Tony Cliff's positions.
Hallas was a founder member of the tiny Socialist Review Group when it was organised in 1951 and wrote its only major founding document not authored by Cliff, "On the Stalinist Parties".
With the upsurge in left-wing political activity in 1968 Hallas joined the International Socialists (IS) and rapidly became a member of the group's leadership and a full-time worker at its headquarters.
He remained within the IS when the ISO's members were expelled, becoming a leading figure in its successor organisation, the Socialist Workers Party, until his retirement from active politics, owing to ill health, in 1995.