Origins of the 'Special Relationship' were decidedly one-sided... and not in favour of UK

Winston Churchill and Franklin D Roosevelt forged a war-winning alliance, and a deep personal friendship that survived many crises. American military power and huge quantities of economic aid were critical in every theatre of the war and boosted both Britain and the Soviet Union.

Securing that support was rightly the focus of Churchill's government from the first hour that he took office. But it wasn't always easy - it was dogged by clashing agendas and deep suspicions and it very nearly didn't happen at all. Because although it may seem obvious now that the US would ultimately come to the aid of Britain, it seemed anything but that during the first year of the war. Helping Britain resist Europe's tyrants was very much Roosevelt's intention but this was not an easy sell to his party or his nation.

Anti-war, anti-British and anti-colonial feelings were strong in America and there were powerful groups − notably the many voters of Irish, German and Italian descent − who did not easily give their support to Winston Churchill and the nation that he led. Powerful figures like the US Ambassador to Britain, Joseph Kennedy, and US isolationist crusader (and borderline pro-Nazi) Charles Lindberg, had mass appeal, as can be seen by the huge scale of the latter's anti-war rallies.

Equally, there was a presidential election coming in November 1940 and so, if Roosevelt wanted to help "the Limeys", he would have to show his people and his party there was something in it for Uncle Sam.

He did so by extracting some valuable real estate in return for US aid through the "destroyers-for-bases" deal in September of that year, where 50 US Navy warships were transferred to the Royal Navy in exchange for land rights on British possessions. And then - having won re-election - he was able to go further with the Lend-Lease scheme, in which he could lend or lease war materials to allies, including Britain.

Winston Churchill was as "excited as a schoolboy on the last day of term". Accompanied by a "retinue which Cardinal Wolsey might have envied" - as his Assistant Private Secretary, Jock Colville noted - he travelled north on August 3, 1941, from London to Thurso, writes Tim Bouverie. There, he boarded a destroyer and thence the HMS Prince of Wales for the four-day voyage to Newfoundland and his rendezvous with the US president.

The meeting between Churchill and Franklin D Roosevelt at Placentia Bay from August 9-12, 1941, was the first major wartime conference. These became crucial to the prosecution of the war, Allied unity and development of the post-war world - and involved considerable risk.

"What a target we were - Winston Churchill and the Chiefs of Staff!" wrote an accompanying journalist as the battleship cut its way through the U-boat-infested waters of the Atlantic.

Churchill would cross "the pond" a further five times during the war, while in August 1942 he flew to Moscow, via Cairo and Tehran. General Douglas McArthur - no Anglophile - thought the PM deserved the Victoria Cross for this journey alone. Yet the wartime conferences were often acrimonious. The Americans suspected the British of wanting to delay the liberation of France.

At Tehran in November 1943 they sided with the Soviets to force Churchill to commit to a spring date. On other occasions, Britain and America clashed on the conduct of the war in the Pacific, the future of the Empire, atomic research and financial support.

Yet there were also broad areas of agreement and genuine comradeship. The Anglo-American military alliance, as established by Churchill and Roosevelt, was the most successful coalition in history. Relations with Stalin were less easy.

At Tehran and again at Yalta in February 1945, Churchill, and to a a lesser extent Roosevelt, battled to save Poland from Soviet domination. But Stalin was unbending. With his forces in control of the country, he refused to countenance any move towards Polish autonomy.

The wartime conferences showed the Allies at their best and worst. Churchill and Roosevelt flattered themselves that they could influence Stalin. But the paranoid despot was not easily managed. The war in Europe ended on May 8, 1945, but the 'Iron Curtain', as Churchill later dubbed it, had already begun its slow descent.

• Tim Bouverie's new book, Allies At War: The Politics of Defeating Hitler (Bodley Head, £22), is out now

However he still needed a major gesture from Churchill and, in August 1941, he got it - a written promise in the form of The Atlantic Charter that the age of imperialism was coming to an end. This, however, was very much not how the document was interpreted in London, where Churchill insisted: "We must regard this as an interim and partial statement of war aims designed to reassure all countries of our righteous purpose."

Tensions over the fate of British colonies would dog the relationship in the years after the charter was signed. The most senior American military leaders were always quick to question Britain's true motives. Why, for example, did Churchill resist the "Second Front" - as the planned invasion of France was known - for so long? Did he prefer to let Hitler and Stalin exhaust themselves while he looked after Britain's imperial interests in the Middle and Far East?

The attitudes of both sides were affected by popular stereotypes. America: brash and over-confident, liable to charge headlong into military disaster, unappreciative of British sacrifice. Britain: cagey, deceptive, keen to win the war with American dollars and Soviet blood. As Roosevelt himself once privately said to his cabinet: "The English promised the Russians two divisions. They failed. They promised them help in the Caucasus.

They failed. Every promise the English have made to the Russians, they have fallen down on... Nothing would be worse than to have the Russians collapse... I would rather lose New Zealand, Australia or anything else than have the Russians collapse." And - they wondered in Washington - why did the British want to retake Burma, Singapore, Borneo and Malaya rather than help the Chinese stay in the war against Japan, which was America's priority in the Far East? Was it to prioritise rubber plantations, oil fields and old-school imperial flag-waving over a sensible war-winning strategy?

The other great source of tension was money, as Britain's enemies well understood. Nazi propaganda aimed at British troops portrayed them as dupes, risking their lives whilst the people who were really behind this war lived it up at home, most likely in the company of their wives or girlfriends.

The seducers came in two stock caricatures: Jewish-looking American businessmen and big spending American soldiers, both laughing up their sleeves at the gullibility of anyone who believed in all Churchill's guff about democracy. The way the German stations told it, GIs were screwing Britain's women, Wall Street was screwing her economy and the White House was screwing her empire. And, like all the best propaganda, it hit on a weak point because some of it was true.

Roosevelt's hard-headed team wanted to tear up the pre-war economic and financial system - to America's benefit. This threatened the future of the British Empire, which existed behind the tariff walls of the Imperial Preference - a system of mutual tariff reduction.

Secretary of State Cordell Hull, Treasury Secretary Henry Morgenthau and Vice-President Henry Wallace were all veteran "New Dealers" from the 1930s, promising reform to Americans. They believed the war provided the country an opportunity to "set a sick world straight" once and for all, which meant sweeping away all the old trade barriers. The British economist John Maynard Keynes spent most of the war negotiating Britain's financial relationship with men like this. He soon discovered that however polite Roosevelt was to Churchill, the idea of removing imperial tariffs was regarded as a "neo-religious quest" in Washington and pursued with passion.

After much arm-twisting and agonising, Keynes's final deal included a specific promise to eliminate "all forms of discriminatory treatment in international commerce". Churchill wrote an angry note to Roosevelt to complain that Britain was not being treated like an ally but "a client receiving help from a generous patron". Then he thought better of the message and never sent it. The view from London went like this: Britain had already bankrupted herself to fight this war - a war that was in America's interest. Why should she be further penalised by onerous financial and trade constraints?

But from Washington the issue looked very different. This was a war to create world freedom, not reinforce the stupidities of the past. Two wars and one traumatic depression had been caused by European rivalries and the closed trading systems of the old empires. Since America was paying for everything now, why should she be expected to subsidise a failed system?

Because of his genuinely warm relationship with Churchill, Roosevelt himself may never have said any of this openly. But Harry Truman, the man who replaced the president after his death in April 1945, was nothing like as sentimental when it came to advancing American interests.

After months battering away at men like Hull and Wallace, an exhausted Keynes concluded that one of America's principal war aims was to liquidate the British Empire. The Americans were using Lend-Lease as a means of destroying the whole financial and trading system that Britain had built on the foundations of the sterling area and imperial preference. It was Keynes's opinion that America intended to treat Britain "worse than we have ever ourselves thought it proper to treat the humblest and least respectable Balkan country".

Holding Britain's cash and gold reserves at a historically low level was official US government policy as early as 1942. This meant Britain was fated to start the post-war era with a giant balance of payments deficit and financial reserves that were billions of pounds smaller than its debts. In London there had always been those who thought that the war was a great mistake.

On the back benches of the Conservative party, hardcore appeasers such as William Greene were heard grumbling that "we should have backed the other side", and as recently as 1940 a sizeable proportion of Churchill's cabinet was unsure about fighting on. Clear-eyed men like Foreign Secretary Lord Halifax could see that to continue fighting would entail economic ruin, even if Britain won. But its priorities were changing too. Its propaganda emphasised human rights rather than imperial might. That alone suggested the slender resources of post-war Britain would be spent on council houses and hospitals rather than battleships.

Some historians still think that Churchill got it wrong. In a phrase that evokes the old Berlin radio commentaries, one concluded that in fighting the Second World War, "the British had certainly gained a national myth, but they had also ensured they would need one". However, there was really only one other path available, and it led to disgrace.

Her ambiguous relationship with America cost Britain much of her economic independence, as became painfully clear in 1945, but it allowed her to preserve something rather more important: her real independence. Without those planes and tanks and lorries, without all those convoys of food and oil, she could so easily have become a second Vichy France.

• Phil Craig is the author of 1945:Reckoning - War, Empire and the Struggle for a New World (Hodder, £25)