Q:

Late 80s/early 90s DC Comics arc with Greek (and Roman?) gods involved?

In the late 80s or early 90s, DC put out another one of those story arcs where basically everyone in their universe was involved (a DC staple, methinks), and in this particular arc the ancient, classical deities and characters and their world was either merged with or came down the earth. 
I can't remember if they were Greek gods or their Roman counterparts (or possibly both?), but I remember it was a huge crisis (duh, it's DC!) and scenarios put DC heroes in situations where they were matched up against similar deities (like Flash having to run against Hermes or Mercury).
I also can't remember if it was a massive crossover or a limited series. I do remember having several of the comics, though, and I lean towards thinking it was a mainly a limited series but ongoing titles had tangential crossover stories. 
My son is really interested in mythology and comics and I think he'd dig this, so thanks in advance for any help.

A:

You are probably thinking of War of the Gods, a mini-series that came out in 1991.  It was centered around Wonder Woman, however it did involve a majority of the DC's earth based heroes.  The mini did have crossover issues of most of DC's top tier books, such as Batman, New Titans and Superman.  It was written and drawnn by George Perez, and was intended to celebrate Wonder Woman's 50th anniversary, however due to editorial conflicts (which ultimatley lead to Perez leaving Wonder Woman) it failed in Perez's eyes to have the impact it deserved.
A part of the main conflict of the story invloved the Roman gods doing battle with the Greek pantheon, which is probably what you're thinking of.

