Diplomacy? Depends; are you referring to overall league diplomacy or individual player diplomacy?
Those in strong leagues often rely overmuch on their league rep to scare people away from attacking/retaliating. I find most leagues will look the other way when you can show them your attack is justified.
As I'm not high enough ranking to deal with league diplomacy, I'm not sure how that would go. I'd assume it is based on what people think your league is capable of. Having players with a reputation for being very strong, doing well in league tournaments, etc help build a perception that you can kick some serious posterior when you want to. The stronger they think you are, the more likely they are to play nice with you. Of course, there are other factors as well. A league preparing to take beacons may want to avoid trouble, even with leagues they could easily bury.
With players, diplomacy really varies wildly depending on their reason for hostility. Some is pretty straight forward. Don't attack my holdings and I won't kick your posterior. Other times, it is less so "I raid you because I can and there's nothing you can do about it" "Nothing personal, just hitting for the PvP". Sometimes you can negotiate, other times, you can't. When you can't negotiate, the best thing to do is be an unattractive target. Most will leave you alone when they get very little for hitting you. If you have an honorless knave fallen from grace, someone who attacks not for any in game gain, but just to harass and make your life miserable, you might be better off moving to another location or finding friends who can help you beat them.
Honored Sir, my Marshall has sent me forward to seek peace if possible, friendship where we may, expecting that I, and my men show honor to reflect honor on him. I ask in respect, that we be given safe passage, letting you know in advance, my Lord Marshall, is Powerful, and caring of his people. Your actions, towards us, his emissaries, will speak far more loudly than written letters.
in respect
Dek
Proudly
AOC