they mention letting Martian Manhunter on the team and it ending up badly. Did this happen in a separate issue in another title or is this going to be explained later on?

That's only shown in that issue and much probably wouldn't be explained

@matchesmalone21: ah i see. thanks :)

@martinceld: But in Stormwatch 4 (I believe) Martian Manhunter mentioned that instance too,he joined Justice League to know if they're prepared to incoming threat,seeing they're not ready leaves and join Stormwatch.