In 1914, World War One broke out in Europe. This conflict included the Triple Entente of Britain, France and Russia and their enemies Germany, Austria-Hungary and the Ottoman Empire (otherwise modern-day Turkey). By 1915, the Western Front had become static with few advances made by either side. As such, upon the suggestion of First Lord of the Admiralty, W Churchill, the Entente switched their attention to Gallipoli and opening a front with Ottoman Turkey.
The Dardanelles was a strategically vital sea that separates Europe from Asia. If Britain and France could defeat the Ottoman Empire's defenses along the Dardanelles, and gain access to that vital stretch of sea, they could potentially advance towards the capital of Constantinople. Taking Constantinople would have defeated the Ottoman Empire.
Other strategic considerations also came into play for the campaign. Firstly, the Entente hoped that Germany would divert more of their troops to support of Turkey if they opened a new front in the Dardanelles. As such, that would weaken the Western Front. By seizing the Dardanelles Strait, Britain and France could set up a supply route to Russia. In addition, they expected that a victorious campaign would win over new Entente allies in the Balkans such as Greece and Bulgaria.
So the Gallipoli Campaign seemed to have great potential. France and Britain did not consider Turkey's army among the best in war, and certainly in terms of equipment and supplies this assumption was not entirely inaccurate. However, the Dardanelles Straits was still heavily defended with fortifications and lots of troops.
Britain and France expected a naval bombardment to break-through the Ottoman Turks' defenses around the Dardanelles. The first stage was effective as outer forts fell to marines after battleship naval bombardments. However, they were in for a shock when naval bombardments by British and French warships began to falter. They soon came under heavy artillery fire from the Turkish artillery positions. Worse followed when they headed into the heavily mined waters of The Narrows and lost further warships. The artillery and mines ravaged five British and French warships.
Consequently, it became clear that the campaign could not rely solely on navies if the Entente was to secure the Dardanelles; and that ground support was required. So the Entente landed their armies in three positions on small beaches in the south, the ANZAC (Australian and New Zealand Army Corps) in the north and French troops at Kum Kale. Those armies established three beachheads at ANZAC Cove, Sulva Bay and Helles.
During these landings, the Entente had high casualties. Most of the advances failed to break Turkey's defenses that held the high ground. As the campaign continued, Entente opposition to it increased. The failure of Hamilton's forces to break out from Sulva Bay and link up with the troops at ANZAC Cove all but ended the Gallipoli Campaign, as their was little prospect of further reinforcements arriving in the Dardanelles. An invasion of Serbia and plans for landings at Salonika exhausted Entente resources.
Then Britain and France evacuated their armies from the area. Fortunately, that evacuation of remaining troops was more effective as there were no further losses when the Entente retreated. Thus, soldiers were saved at the end of the campaign.
However, the Gallipoli Campaign was a defeat. The Entente had hundreds of thousands of casualties in a campaign that failed to reach its objectives, and ANZAC Day is now a day of remembrance in Australia and New Zealand. It was a last great victory for the Ottoman Empire that would nonetheless be defeated in World War One after the Lawrence of Arabia-inspired Arab revolt.