Wonder Woman has fought many battles with Ares (Mars), but which was the comic where she first encountered Ares and how she defeated him ?
The first time Ares (formerly known as Mars) fights Wonder Woman (albeit indirectly) is in Wonder Woman Vol 1 #1 from June, 1942.
Ares can be seen behind Wonder Woman's head, with the speech bubble. The next panel confirms Mars' (Ares') defeat to Wonder Woman. Where she beats them by using her lasso to tilt the space ship and knock them off.
Finally, the first time Ares fights Wonder Woman proper, is in Wonder Woman Vol 1 #5 from July, 1943
A fight which Wonder Woman wins by knock out.