Gallipoli, in modern day Turkey, is a peninsula that forms the northern bank of the Dardanelles, one of the only passages between the Black Sea and the Mediterranean. As such, the Dardanelles has always been of great importance from a military point of view, and remains strategically relevant to this day. Control over the area has been the objective of a number of hostilities, notably the attack of the Allied forces on the Dardanelles during the 1915 Battle of Gallipoli in the course of World War One.
After the Ottoman Empire entered into the war during August 1914, Winston Churchill, then First Lord of The Admiralty (the political figurehead of the Royal Navy), formulated a plan for a naval attack on The Dardanelles with the ultimate goal of a move on Constantinople. Although the plan was authorised, it relied heavily on intelligece that proved to be flawed. To compound matters, the navy could only spare out-dated warships and the operation was ultimately unsuccessful, with three battleships sunk and the loss of 700 lives.
"The price to be paid in taking Gallipoli would no doubt be heavy, but there would be no more war with Turkey. A good army of 50,000 and sea-power—that is the end of the Turkish menace."
Allied leaders subsequently decided that in order to secure the straits, Turkish artillery would need to be eliminated and a land attack was necessary. A force of British, French and the newly created Australia and New Zealand Army Corps (ANZAC) soldiers was formed and readied for an assault on 25th April 1915. However intelligence for the attack was again poor and the Allies were taken by surprise, both by the severity of the terrain and the strength of the Turkish forces lying in wait. In addition the landings themselves were badly mismanaged by commanding officers and the Allies suffered very heavy casualties. The landings proved to be a slaughter and were later referred to as the first D-Day.
A protracted and bitter episode of trench warfare ensued, with attacks and counter attacks from both sides leading to no real ground being gained. Losses were heavy, with dead soldiers from both armies lying together, putrefying in the blazing summer sun. Ultimately, after eight months of fighting, the conflict had become a stalemate. As time wore on, conditions steadily worsened for the Allies and many men suffered from lack of food and water, exposure to the elements, and disease. With no end in sight, in December of 1915 it was decided to begin an evacuation of the peninsula.
By the end of the campaign the Allies had over 302,000 casualties with numbers for the Turkish forces reaching 250,000. Winston Churchill’s involvement as one of the architects of the disastrous mission saw him demoted and he ultimately resigned from government to enlist in the army where he became an officer serving on the western front.
This website features a transcript and scans of the war diary of Jack Lock who entered the Gallipoli Campaign with the 4th Battalion Norfolk Regiment in August 1915. His Battalion was involved in a second wave of landings in that month that were intended to bolster Allied numbers and to keep pressing for an ever less attainable victory. He records his experiences of the conflict in vivid detail in the pages of his diary, the transcript of which can be read here.
More information can be found on many sites across the web, but perhaps a good place to start is the wikipedia article about the campaign. There are also several exellent documentaries available, one of which is shared below.