Explanation: In a war filled with pointless and bloody campaigns, Gallipoli stands out as one of the worse incidents of a pretty fucking terrible war. In WW1, the British happened upon the 'brilliant' idea of making a strike directly at the Ottoman Empire's heartlands in order to knock them out of the war. ANZAC troops - soldiers used by the British Empire from Australia and New Zealand - were gathered as the main force for this attack. Pushed in no small part by Winston Churchill, at the time First Lord of the Admirality, the campaign anticipated taking Istanbul in two weeks. The Ottoman military was considered to be no match for British troops, and would provide only marginal resistance. Back home nearly in time for tea, eh?
Things didn't go as planned. The initial naval assault was borked, but the landings went ahead anyway along rugged coastal terrain. What's worse, one of the greatest officers of the Ottoman side happened to be stationed at Gallipoli - Mustafa Kemal, later known as Ataturk. Though just a colonel at the time, he was intimately familiar with the geography of Gallipoli, and correctly predicted the landing dispositions of British forces.
This meant that when ANZAC troops came ashore, they didn't get much further than the shore - zeroed in artillery and concentrated Ottoman troops made the initial landings a massacre. What was meant to be a quick and morale-boosting strike turned into a horrific meat grinder, wherein well-equipped ANZAC troops with naval and air support were shoved head-first into well-prepared defensive Ottoman positions manned by determined defenders. The illusion of sweeping away the 'paper tiger' of Ottoman forces and taking Istanbul quickly faded, and the two-week operation turned into a 8-month catastrophe.
The ANZAC forces acquitted themselves extremely well under the circumstances, but the resentment of being used as meat for the grinder, especially with the perception that they were used to keep 'real' British losses low, led to an increased sense of national identity apart from the UK in Australia and New Zealand. On top of that, both the Ottoman forces and the ANZAC troops regarded each other highly, both in terms of skill and on a human level - the absurdity of the whole conflict came out during short truces, creating questions in ANZAC forces of what the fuck they were even doing there.
Eventually, the British forces would withdraw, having accomplished none of their major objectives, giving the Ottoman Empire one of their biggest victories of WW1.
Mustafa Kemal would forge his initial reputation as a war hero in the defense of Gallipoli, and go on to become the first president of the Turkish Republic. A quote of his (which may have been stitched together from multiple speeches, as it's first attested to in full form only after his death) highlights the senselessness and loss, but also the common humanity even in such a brutally contested campaign.
Those heroes that shed their blood and lost their lives. You are now lying in the soil of a friendly country. Therefore rest in peace. There is no difference between the Johnnies and the Mehmets to us where they lie side by side in this country of ours. You, the mothers, who sent their sons from far away countries, wipe away your tears. Your sons are now lying in our bosom and are in peace. After having lost their lives on this land they have become our sons as well.