This is such a broad question because the Allies took so many actions during the war. Perhaps you could let us know which actions you would like us to discuss.
For example, we could ask why the Allies used poison gas in the fighting. In that case, the defense is fairly simple. They used it because the Germans used it first and the Allies did not want to have fewer weapons than the Germans did.
Or we might ask why the Allies chose to go to war at all. If that's what we are asking about, they would say that they were forced to go to war because Germany was becoming too powerful. They could not let Germany fight Russia without helping Russia because that might let Germany get too strong. It would also have meant abandoning Russia and France did not want to do that since they had been allies for decades.
There are many other things that could be discussed. Which actions are you most interested in?