"The Defector", TNG. A superb scene in Picard's Ready Room:
PICARD: You see, I'm just not convinced that you are telling the truth.
JAROK (exasperated): What must I do?
PICARD: You must convince me. If I had irrefutable evidence? But you did not bring irrefutable evidence. You brought no evidence at all. Now, here, you are not the man you claimed to be. Admiral, your credibility is stretched beyond belief. A Romulan defector is almost a contradiction in terms. But Admiral Jarok crossing the lines?
JAROK: I explained my motives to your interrogators.
PICARD (dismissive): Yes, yes, yes, yes, yes. Peace in our galaxy. Except, Admiral, you are not a man of peace. Your military record, what we know of it, is clear.
JAROK: Which is precisely why I chose an alternate identity here.
PICARD: The massacres in the Norkan outposts, for example.
JAROK: What you call massacres were called the Norkan Campaign on my world, Captain. One world's butcher is another world's hero. Perhaps I am neither one.
PICARD (shaking his head): On what basis, Admiral, am I to decide? Your good word? Are you willing to help us overpower the Romulan B-type warbirds we may encounter? Are you prepared to help us detect them through their cloaking shields?
(Jarok sits silently)
PICARD: You see my problem, Admiral. You ask us for faith in circumstances which are hardly possible to believe, compounded by lies and your refusal to tell us all you know.
JAROK: I cannot betray my people!
PICARD: You've already betrayed your people, Admiral! You've made your choices, sir! You're a traitor! Now, if the bitter taste of that is unpalatable to you, I am truly sorry. But I will not risk the lives of my crew because you think you can dance on the edge of the Neutral Zone. You've crossed over, Admiral. You make yourself comfortable with that.