What was the most demoralizing experience suffered by the Allied Powers during World War One?

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The First World War had many of these but in my opinion the Top Ten would have to be

1. The Treaty of Brest Litovsk that ended Russian participation in World War One.
2. The great loss of manpower at the Somme (Allied loss over 600,000….57,470 British casualties on the first day alone)
3. The tremendous loss of life at Passchendaele (for hardly any territorial gain)
4. The German tactical victory of the Spring Offensive (although it would turn out to be an Allied strategic victory)
5. The Failed Gallipoli campaign (directed against the Ottomans)
6. German victory over the Russians at Tannenberg (1914)
7. The largely failed or limited success of the Italian forces during the many Battles of the Isonzo
8. The vast amount of Allied shipping lost to German U Boat attacks in the Atlantic
9. Battle of Loos (1915)
10. The Nivelle Offensive and the French Armed mutiny.