The US president, his forces hammered by the Confederate army, turned in desperation to Garibaldi, spawning one of the great what ifs of history.

Rumours of Lincoln's offer have circulated for a century and been denied by American scholars, but the document proved it was no myth, said Arrigo Petacco, a historian.

He stumbled across the faded blue postcard, from Garibaldi to King Victor Emmanuel II telling the king of the offer, last week while rummaging in 90 boxes of material donated by Italy's exiled royal family.

Garibaldi caught the world's imagination in 1860 after invading Sicily with 1,000 lightly armed redshirts. They defeated 12,000 Neapolitan troops, took the island and, determined to unify the Italian peninsula, invaded the mainland. They occupied Naples and unleashed a wave of support.

According to Mr Petacco, the rebel, who in the 1850s had led an army in Uruguay and travelled through the US, was also a mason. The international masonic lodge successfully lobbied for him to be granted American citizenship.

Garibaldi was ready to accept Lincoln's 1862 offer but on one condition, said Mr Petacco: that the war's objective be declared as the abolition of slavery. But at that stage Lincoln was unwilling to make such a statement lest he worsen an agricultural crisis.

"Later they offered Garibaldi the command of one unit, rather than the whole army, but at that point it was too late and he had gone on to do other things," Mr Petacco said. "In Italy we always knew, but there was always a lot of scepticism in America. Now we know for sure."