My theory on the film then..
Ares is the main villain. And is growing in strength with the Amazons recently defeating some of his forces whom they fought long destroyed.
Around the same time, World War I starts.
Captain Steve Trevor crash lands on Themyscira and meets Diana. The Amazons realize the war is feeding Ares' strength and must be stopped. Diana goes back to Mans World with Trevor to end the war and Ares.
Diana goes in disguise as Diana Prince, who works in the British Army and there is like a group led by Trevor including a few British Officers and Diana, with 1 French guy. These are the main protagonists we follow through the film, fighting in the war whilst trying to find Ares. Trevor tries to convince the others that Diana is immortal and that Ares and all the other "myths" exist.