There were two Dardanelles campaigns during WWI. The first one, in 1914, was to try to secure them as fast as possible, but a German fleet as well as turkish guns made it too hard.
A second campaign, the so-called Gallipoli campaign, was a real fiasco, but it's primary objective was to "create a diversion", or a second front to help the Russians (that's why they stayed so long on Gallipoli). At this point of the war the Dardenelles were heavily defended and minefields were everywhere.
The naval part of this French-English operation was poorly conducted. The basical strategy was to go head first into the landmine, to get close enough to the cost guns to effectively target them and destroy them. The minefields as well as the turkish guns and the level of fortification had been totally underestimated, so the first stages took more time than expected, with minor results.
The land part of the operation was started before any minefields could be removed, and before the coast defences could be weakende enough. It was a fiasco, and soon a second plan was decided, a deployment on the "Gallipoli" peninsula, defended by a colonel later known as Mustapha Kemal Atatürk. The landing and the battles on the peninsula were a success. The further operations involved massive support from England and France, which never came, because of the ongoing slaughter in France. As the month passed, the Turks and the Allied dug in, and the Turks got heavy artillery support and massive reinforcements. So the evacuation was decided, and it was decided to try to push through the Balkans.
The Evacuation was perfectly carried out, which makes this operation a very good example for landing operations. Churchill considered the operation could have been a success if the reinforcements had arrived (as he wrote to Admiral Guépratte later). They never came.