Swizz Beatz -- Ex-Wife & Alicia Keys Call Truce ... After Homewrecker War

It's the most unlikely truce in history -- Alicia Keys and Swizz Beatz' ex-wife Mashonda ... the same woman who once publicly accused Alicia of breaking apart her marriage.

The photos were taken this weekend in St. Barts -- showing Swizz, Alicia, and Mashonda all vacationing together with their children (Swizz fathered kids with both women).

It's wild -- nearly 5 years ago, Mashonda wrote a nasty open letter to Alicia in the wake of her divorce from Swizz, blaming Alicia for taking part in the affair that destroyed her marriage and family.

From the letter, you'd think Mashonda and Alicia would be enemies for life -- but the photos tell a much different story.

Swizz Beatz -- the new Simon Cowell.