IT is the first written indication of what would become an intense rivalry between Michael Collins and Eamon de Valera, ultimately splitting the nation.
It is expected to fetch up to €50,000 at auction next week.
A letter, written by Collins on the first anniversary of the Easter Rising in April 1917 to fellow Rising leader Thomas Ashe, gives his views of the incredibly tense political affairs of the time.
Written in code -- as Ashe was still in prison -- it contains some caustic remarks about de Valera and Arthur Griffith as Collins went about strengthening his own political base and rebuilding the Irish Republican Brotherhood following his release from internment a few months earlier.
At one point, Collins writes that "the new master is going strongly for the highest salary" -- a reference to de Valera's well-known ambitions.
The pair would continue to work closely together for a number of years -- with Collins masterminding de Valera's escape from Lincoln Prison in 1919 -- but the rift developed steadily and would ultimately culminate in the Civil War.
Supporters of Collins would argue the 'Big Fella' was hung out to dry by de Valera when he signed the Treaty, as de Valera himself did not go to the negotiations with the British, knowing exactly what was on the table.
Supporters of de Valera, in turn, would argue that he has been unfairly demonised by history and negatively portrayed at every turn, famously in the movie 'Michael Collins'.
The letter comes from the family of Ashe, who died on hunger strike later in 1917, and is accompanied by a portrait of him by renowned painter Leo Whelan. The portrait is expected to fetch between €50,000 and €70,000.
Other items up for auction include items of correspondence from Collins and Liam Mellowes, as well as an autograph book from Frongoch camp containing watercolour views of the prison camp and a manuscript poem by Collins.
The auction starts at 6pm on Monday, November 26, at the RDS.