The court martial convened after the USS Stargazer was abandoned at Maxia, in which prosecutor Phillipa Louvois pursued Jean-Luc Picard with a zeal he never quite forgave, though he was cleared.
A court martial is standard Starfleet procedure whenever a starship is lost. After Picard abandoned the crippled USS Stargazer following the Ferengi attack at Maxia in 2355, he was duly tried, with Phillipa Louvois of the Judge Advocate General's office acting as prosecutor.
Picard was exonerated, but he felt Louvois had gone far beyond what duty required, and the experience ended their personal relationship. She left Starfleet for a time afterwards.
The trial is never shown. It is recalled in The Next Generation episode The Measure of a Man, when the two meet again at Starbase 173 a decade later and Louvois, now a captain heading the sector's new JAG office, presides over the hearing on Data's legal status.