Picard: 15 ESSENTIAL Star Trek Episodes To Watch First (And Why)
4. “Inter Arma Enim Silent Leges" (Deep Space Nine - Season 7, Episode 16)
There's limited reason to watch Deep Space Nine in the run-up to Picard, unless the show does the right thing and brings in Worf secretly as a main character or something, but you should never miss an opportunity to watch more of the most unfairly underrated Star Trek show of all.
This episode is very much relevant to understanding the Romulans, though, as it explores the grubby world of the Romulan secret police, the Tal Shiar, and the Federation’s Section 31. It's an episode focused on an elaborate, shady operation to ensure the Romulans stay in alliance with the Federation and really gives good insight into the more morally ambiguous areas Starfleet sometimes operate in.
Given Picard has to go off-the-radar for his unauthorised mission in Picard, it's an intriguing point to explore a little more.