If Gallipoli had opened the Dardanelles

What might have changed if Gallipoli had captured the Dardanelles?

The central effect would be renewed supply to Russia, which could keep it fighting longer and place much greater pressure on Germany's two fronts.

Opening the Dardanelles would restore a route for supplying Russia through usable ports. Better supplies could keep Russia fighting and make Germany's war on two fronts harder to sustain. Russia's first revolution is still considered likely, but the second, Bolshevik revolution becomes less certain in this scenario. Germany might then lose early enough that the United States had no opportunity to enter the war. A Russia that remained in the war could also take part in the Versailles settlement and the Washington Naval Treaty system. These are possible consequences of the counterfactual, with Russian supply and political endurance connecting the campaign to the post-war settlement.