Victoria 3 diplomatic plays are the openings for most important international actions - they're hostile demands that can often result in war. It's a new concept for Paradox grand strategy games, but don't worry: the diplomatic play system is easy to use, and in most cases, these situations will handle themselves.
During diplomatic plays, one nation is making a formal demand of another, and any nations interested in the region where this is taking place have the opportunity to weigh in and sway the outcome. Knowing what's happening and when you can take action is key to using and responding to diplomatic plays effectively.
RELATED LINKS:
How Victoria 3 trade works and how to start a new trade route
Victoria 3 review - a golden tomorrow
Victoria 3 impressions: Building the future