57. Mars is destroyed (Justice League of America #71, 1969, collected in Showcase Presents: Justice League of America Volume 4)
The history of Mars in the DC univsere hasn't been a stable one. It started out peaceful in J'onn's earliest appearances, and then we learned that it lived up to its name and was stricken with a world-wide civil war, before being changed back to mostly peaceful again. (I think I've got all the bases covered...maybe.)
Anyway, during the original run of the Justice League of America, J'onn came back to enlist the help of his JLA friends to save his home planet from a greedy pale-skinned Martian warlord named Commander Blanx.
As is oft with a Martian Manhunter story, its is steeped in tragedy, and there is little in the way of a happy ending. J'onn and the JLA fail to stop Blanx before he incinerates the whole planet.
The good news is that Blanx is vanquished, but it is little consolation after the destruction of a world.