The Alexandre Millerand Letter was written by French Prime Minister Alexandre Millerand and was an annex to the Trianon Treaty. It stated that the border amendments made by the Trianon Treaty were not final, and could be reviewed in the future. This give the Hungarian government the false impression that the treaty itself was only temporary, and so the delegation signed the treaty on June 4, 1920.
Millerand letter Wikipedia
On March 8, 1920, the Council of Foreign Ministers and Ambassadors discussed for the last time the new borders of Hungary in London, and Millerand was the chairman. Lloyd George questioned the viability of such border modifications, but Philippe Berthelot refused to do any adjustment at the last moment, so as a compromise, the council produced the Millerand Letter, as an annex to the Treaty. The letter stated: "in order to preempt any further discussion regarding territory, that the eventual modification of the designated boundaries would be referred to the boundary committees in accordance with the same conditions".
The letter stated: "Examinations are to be done locally, and in some cases it might be necessary to move the borders stated in the treaty... if the border committees ... come to the conclusion ... that the declarations of the treaty are unfair, then they will have the possibility to report it to the Council of the League of Nations. In this case the Entente Powers agree to that, on request to one party involved, the League of Nations will offer its services to retain the original border, with the same conditions, in a peaceful way, in locations where the Council decides so."
The consequences of this annex were adverse for Hungary. It fed Hungarian revisionism and speeded up the signing and ratification of the Treaty both in the Hungarian Parliament and in England and France.
Léon Blum, politician of the French Socialist Party, published in the French newspaper Populaire that the cost of the help of Millerand would be to put the Hungarian State Railways rented to a French investment fund. According to the publication, if the railway was controlled by this fund, then Millerand would modify the borders in favour of Hungary. Millerand did not respond to this allegation, and it cannot be verified. It is fact that Millerand has receded to do any favour for Hungary afterward.
The Trianon borders were later revised during the Munich Agreement (1938), during both Vienna Awards (1938 and 1940), and reconquering areas from Yugoslavia in 1941. All border modifications were resettled in 1947 according to the original treaty.