And in the lightyears and lightyears that make up space, it would be a little harder to protect a border in space than a border in anything on Civ. Really, you would be able to fly right through a space 'border' because maintaining such a thing would be astronomically difficult.
Tell that to the romulans! hehehe
Well except for China no one has been able to really considerably close their borders to anyone else. OF COURSE watch how hostile AI get's when your on their territory. Even when they can't kick your arse they get mad.
Yea look at Iran for example. What would happen if Iranian sailers wandered into British territorial waters? A british patroll boat would advise them of their mistake and send them on their way. And even if they were detained, would the british people go and protest for them to be hanged? No of course not, because those Iranian soldiers never did anything to hurt the people of Britian just the same as those british sailors never did anything to hurt the people of iran. But those redneck assehole iranian people protest to hang them, it made me sick!