At early dawn on the 25th April 1915, French, British and dominion troops were delivered by small boats to storm the shores of the Turkish peninsular with the objective of invading Turkey and forcing that country out of the war and causing the end of the Great War by many months.
The operation was a complete disaster. The troops were landed not on the expected flat beaches but up against cliffs. The Turks were prior informed and the invading soldiers were shot to hell. As an exercise in military blundering and incompetence the entire Dardanelles campaign could never be 'bettered'.
In the months that followed a grisly total of 27,000 French soldiers were killed.
115,000 British and dominion soldiers were killed this included the New Zealanders, Irish, Indian and many other 'British' countries and 8,709 Australians killed storming their sector a tiny spot on the Turkish map we know as Gallipoli.
Eventually the British high command realized that the Dardanelles campaign was UN-winable and organized a retreat back by sea. This was the only episode of the whole that was a success. Not one more life was lost. This was a miracle.
This death toll was an overture, compared to the rest of the Great War 1914-1918 in which Australia lost 61,522 soldiers.
Hence the 25th of April is the day Australia and New Zealand commemorate their war dead.