When news of the genocide of the Armenians by the Turks reached them, the Allies, on 24 May 1915, issued a joint declaration: "In view of these new crimes of Turkey against humanity and civilisation ... the Allied governments ... will hold personally responsible ... all members of the Ottoman government and those of their agents who are implicated...." Note the words: "crimes ... against humanity"! Thus it was recognised even then that genocide against one people was crime against all peoples.
When the First World War was over, there were the spoils of the Ottoman Empire to fight over, and it was not feasible to implement what became known (and almost forgotten) as the "Declaration of May 24, 1915". Yet some people did remember, and that declaration created the concept that served as the legal yardstick to prosecute the top strata of the Nazi leadership at Nuremberg and formed the core of the Preamble to the Prevention and Punishment Convention of the UN - the very convention that Ignatieff cries over. The idea of retributive justice for the Armenians gave way to the political expediency of appeasing Turkey. Thus, by not punishing Turkey for the genocide that resulted in the elimination of 2 million Armenians from their 3,000-year-old Anatolian homeland, the Allies continued the climate of impunity that existed before and continues, basically, to this day. Impunity creates immunity.
Had the Turks been punished for their deeds, perhaps, just perhaps, Adolf Hitler would not have been prompted to ask, in 1939, "Who remembers the Armenians?.... The world knows only success". Why should any despot (with genocide on his mind) fear the good words of governments and governmentally created organisations when the very activities (or lack of them, in the present discussion) of those governments and those organisations in the past indicate that there will be no implementation of the words?
As any politician will admit, pious principles will always be sacrificed in the name of practical policies.