As late as July 1939, Britain and France had failed to agree to the Soviet Union's requests for an alliance against Germany. On the contrary, the Chamberlain government had pursued a policy of appeasing Hitler and the Nazis, allowing them to annexe Austria and then seize Czechoslovakia. The right-wing Polish government had long rejected Soviet offers of assistance.

Nonetheless, the conclusion of a non-aggression pact between the Soviet Union and Germany in August, which included a secret protocol to divide Poland, came as a shock to anti-fascists, Communists and anti-Communists alike. Keeping out of the war seemed to many Communists a necessary if temporary tactic. It avoided embroilment in imperialist intrigues while allowing the Soviet Union time to prepare economically and militarily for the inevitable war with Nazi Germany, when it came.

When Britain declared war on Hitler's Germany in September 1939, following the blitzkrieg on Poland, the CP at first supported the war while demanding that Chamberlain and the men of Munich be replaced by committed anti-fascists. However, within weeks the Party's central committee reversed this 'fight on two fronts' position, accepting the Comintern’s analysis that this was an “imperialist and unjust war for which the bourgeoisie of all the belligerent states bear equal responsibility. In no country can the working class or the Communist Parties support the war”.

The overwhelming majority of the party leadership backed the change of policy, while the minority—notably Harry Pollitt, JR Campbell and Willie Gallacher—maintained that the earlier position had been correct. In view of their differences, Pollitt was replaced by R. Palme Dutt as general secretary and Campbell by Bill Rust as editor of the Daily Worker.

The majority of Party members also expressed their preference for the anti-war position at a series of well-attended and lively meetings across Britain. 

Many had been confirmed in their view by the flow of events. Spain and Albania had succumbed to the fascists. In France, the Popular Front government had fallen and the country's 70 Communist deputies placed under arrest. Nowhere did the capitalist and landowning classes look like mounting a serious challenge to the fascist powers.

Nor did the British ruling class appear to be any different. The British Empire belied all the fine phrases about freedom and democracy. The declaration of war on Nazi Germany had fulfilled a diplomatic obligation—but it produced no serious military offensive. The Cabinet refused to bomb German arms factories, because they were 'private property'! 

Whereas an ineffective British Expeditionary Force was sent to France to do very little, a 100,000-strong Franco-British army was despatched to assist the right-wing Finnish government in its anti-Soviet endeavours.

Meanwhile, in Britain, bunkers were built for the top bureaucrats, business leaders and politicians, while working class people were left defenceless. Almost alone, the Communist Party campaigned for proper Air Raid Precautions—especially deep underground shelters—for the general public.

The period of 'phoney war' came to end with the blitz on London and other cities from September 1940.    

As dockside communities and those closest to the munitions factories sustained devastating damage, Communist councillors and community leaders such as Phil Piratin and Ted Bramley led mass invasions of the Mayfair and Savoy Hotel shelters. They broke into Liverpool Street, Warren Street, Highgate and Goodge Street underground stations, leading thousands of people to safety, and forcing the government to open them every night.

Above ground, the CP and its allies convened a huge People's Convention in January 1941. More than two thousand labour movement delegates demanded a 'people's government' to secure a 'people's peace', an end to profiteering, nationalisation of key industries, democratic and trade union rights, friendship with the Soviet Union and freedom for Britain's colonies.

One week later, the British coalition government headed by Winston Churchill decided not to outlaw the Communist Party. Labour home secretary Herbert Morrison banned the Daily Worker and The Week instead.

Government and employers faced another problem soon after. An unprecedented wave of strikes by engineering apprentices hit Clydeside, Edinburgh, Barrow, Tyneside and east Lancashire. Some Communists and their allies were prosecuted, although a Court of Inquiry subsequently improved wages and conditions for young workers. 

But by this time, too, the whole character of the war was changing. With the Nazis occupying much of western and central Europe, Communists emerged at the head of popular resistance movements. Then, in June 1941, the fascist powers attacked the Soviet Union.

Quickly restored to the post of CP general secretary, Harry Pollitt declared that 'this is a People's War. One that only the common people can and will win'. For the Party, only the working class could be the driving force in building the national and international unity needed for victory. It had the most to contribute to the anti-fascist struggle, industrially and politically—and the most to lose from a fascist victory.

Britain's Communists threw themselves into the fight for maximum production. While the Party continued to argue for higher pay, its policy was to settle industrial disputes as quickly as possible and establish joint production committees with shopfloor participation. Sometimes, notably in the coal industry, Party members found themselves representing strikers in heated disputes with their comrades in official full-time positions in the union.

The wartime influx of women workers into the factories prompted the CP to step up the fight for trade unionisation and equal pay. The Daily Worker pointed out that, in the Soviet Union, women were perfectly capable of doing so-called 'men's work'. Communist women blazed a trail in the wartime shop stewards movement, campaigning for welfare and nursery facilities.

Initiated through the People's Convention, a Women's Parliament in 1941 sparked a powerful movement for equal pay. When the House of Commons granted it to women teachers three years later, Prime Minister Churchill stopped it by threatening to resign.

An enormous campaign was mounted to lift the ban on the Daily Worker. Just as the TUC was likely to add its support, the paper was restored to legality in August 1942. A new press was bought with £50,000 collected in donations in just three months. Moreover, the wholesalers lifted their boycott so that the Daily Worker could, for the first time, be distributed to retailers alongside the other newspapers.

Orders for the first post-ban issue totalled half a million, but only 100,000 could be produced due to wartime rationing of newsprint. A telegram of greetings was published from the gunners on the George Cross island of Malta, embattled beneath Nazi dive bombers. The paper's front page splashed on the titanic battle of Stalingrad, then approaching its crucial turning point. It also bore the slogan: 'Ten readers for every copy'. A survey later estimated that the Daily Worker had more than half a million daily readers during the Second World War. 

As the Soviet people resisted the Nazi invaders with super-human courage, the Communist Party joined Aneurin Bevan and other Labour left-wingers to demand a 'Second Front'. Britain and the USA should launch an offensive through western Europe as the Soviet Red Army took on four-fifths of the German war machine in the east.

Britain's Communists exposed ruling class motives for delaying the Second Front in the hope that the Soviet Union and Nazi Germany would exhaust each other, thereby enabling the imperialist powers to win the war 'on the cheap' and re-establish their predominance after victory. 

In June 1944, Churchill at last redeemed his pledges to Soviet leader Stalin as D-Day arrived.

The Communist Party's tireless campaigning in the workplaces, local communities and the labour movement helped convince masses of workers that this war—and thereafter the peace—could be won by the working class. Thrilled by the stunning victories of the Red Army, thousands of people in Britain joined the CP. Membership more than doubled, from 17,756 in September 1939 to 45,435 by March 1945.

At the Labour Party conference in spring 1945, Amalgamated Engineering Union president Jack Tanner proposed a policy of 'progressive political unity' for the working class movement—including Communist affiliation to the Labour Party. The CP could no longer be portrayed as an organisation controlled from Moscow—by agreement of its affiliates, the Communist International had been dissolved in June 1943. Against this background, Herbert Morrison's party machine only managed rejection of left unity on a card vote of 1,314,000 to 1,219,000.

With an eye to industrial demobilisation in peacetime, and to joint work with Labour Party organisations in the localities, the CP had reassigned Party factory branch members to residential branches. It proved a costly mistake, losing workplace influence and activists, although the re-establishment of many of the larger factory branches a few years later recovered some of the lost ground.

On May 12 1945, four days after the defeat of fascism in Europe, 500 delegates representing nearly two million organised workers attended a Daily Worker conference on the way forward. Editor Bill Rust spoke of plans to develop 'a front rank national newspaper with a circulation of 500,000 copies daily'. Ownership of the Daily Worker would be transferred from the Communist Party to a cooperative society, thereby guaranteeing 'readers' ownership and control, which is the distinguishing feature of our press'