Well, looking at it in terms or RP, lets take an example:
Outcast meets Corsair (Doesn't matter where for this)
Theese two factions are at war. They need no long winded story to engage, THEY ARE AT WAR!
And as for 'Please leave or die' well, they would not be polite to each other, THEY ARE AT WAR!
An engagement warning is a sort-of sensible idea though, it warns people they're coming under attack so they can defend themselves, incase they do not notice.
Although this in itself, I think is slightly OORP, as the majority of people would not want to miss out on any chance to get an advantage over an opponent, so would not warn them they are engaging. (Only exception is pilots with a large amount of honour, like the Kusarians)