Eamon de Valera twice rejected offers of Irish unity during World War II - but in doing so forfeited any chance of a 32-county Ireland.

In spite of his dream of a reunified Ireland, de Valera rejected the offers because he felt they could not be delivered and were made without consultation with the Stormont government.

Now some historians believe his actions - which ensured Irish neutrality - forged a deeper partition and closed the door to reunification.

An RTE documentary to be screened this week reveals how one of the offers came just five hours after the Japanese bombed Pearl Harbor from Winston Churchill, who was elated that the US would be joining the war effort.

The telegram seemed to offer him the united Ireland he craved if he would join the Allied effort to help crush the Nazis.

But he rejected it as he believed there was no substance to it, and it wasn't worth taking a chance with Ireland's neutrality.

Historian Piers Brendon suggests his stance ultimately cost any chance of reunification.

"The problem was that it reinforced partition and thus made the prospect of a united Ireland that much more impossible - it pushed it out the window, really," he argues.

The documentary traces the fraught relationship between Churchill and de Valera.

"Winston Churchill hated neutrals. He regarded neutrals as being feeble at best and despicably cowardly at worst.

"The chief hate figure for him during the Second World War was 'Devil Eire', as he had taken to calling him," adds Brendon.

The pair clashed repeatedly over partition and the Free State's refusal to join the fight against Hitler.

The seeds of mistrust were planted during the War of Independence.

As Secretary of State for War, Churchill defended the counter-terror tactics of the Black and Tans.

When Churchill hammered out the Treaty with Michael Collins, in which Britain retained control over three ports on the west coast of Ireland, de Valera rejected it, leading to the Civil War.

On September 14 1939, just two weeks after the start of WWII, Sir Neville Chamberlain urged de Valera to join the fight against the Nazis, but the Taoiseach cited partition as the stumbling block.

By May 1940 Hitler invaded France and his U-boats were talking their grisly toll of convoys in the North Atlantic.

Churchill, now head of an all-party coalition, saw access to the Irish sea ports as a top priority.

After rejecting an invasion of Ireland, Churchill decided on diplomacy and sent his minister, Malcolm McDonald, to Dublin to meet de Valera and ask him if Britain could place naval units in Ireland's ports to battle the submarine threat.

De Valera rejected requests for use of ports, but McDonald returned within days with a dramatic proposal that appeared to promise reunification.

However, the fact that the offer was only available 'in principle' provoked strong reservations for de Valera, who, to Churchill's fury, rejected it.

In the documentary, some historians also criticise de Valera for his stance at the end of the war when he paid a visit to the German ambassador in Dublin to express Ireland's sympathy for Hitler's death.

Although many concede that it was the actions of a neutral head of state, others say it was a disastrous move.

- Hidden History - Face Off: De Valera v Churchill, RTE One, Tuesday, 10.15pm.