The Gallipoli War, most often known as the Gallipoli Campaign, happened because the Allied Powers in World War I wanted to control the sea route from Europe to Russia. The campaign was ultimately unsuccessful and lasted from 1915 to 1916.
The Gallipoli Campaign began with a naval attack on the Dardanelles Straits in February-March 1915. This attack was led by the British and French ships. It was soon followed with a land invasion on the Gallipoli Peninsula on April 25th. This involved the British and French as well as the Australian and New Zealand armies.
The Allied Powers wanted the straits because it would allow them to connect with the Russians in the Black Sea. This would help them attack Turkey together and take Turkey out of the War. This would be an attempt to go on the offensive in another region where there was conflict rather than continuing with the same attack offense in Belgium and France. The Ottoman Empire entered World War I in November 1914 on the side of the Central Powers with Germany and Austria-Hungary.
The campaign is considered unsuccessful today because there was not enough intelligence and knowledge of the terrain. There was also strong Turkish resistance. These two factors made it difficult for the campaign to be successful. By mid-October, there were many casualties and little progress to show for said casualties. This resulted in an evacuation in December of 1915. By January 1916, the evacuation was complete and the campaign was abandoned.