The allies tried to regain access to in the battle of gallipoli. alsace and lorraine the suez canal the crimea the dardanelles

The answer is the Dardanelles
The Dardanelles was really important for the allies in order to secure the sea route for the Russian empire which was considered as one of the strongest ally in the war. The campaign (which happened between 17 February 1915 and 9 January 1916) was a major win for the Turkish side and still considered as one of the most treasured event for the nation until this day.

Ithink it is called reservation