I saw that there were passport documents for safety in the past, like you mentioned. I've also heard of documents called passports being used to grant foreigner

I saw that there were passport documents for safety in the past, like you mentioned. I've also heard of documents called passports being used to grant foreigners permission to conduct commerce, and others allowing locals to move between provinces within a country. I wonder about borders, too. I wonder when they started guarding borders like they do today. I know there were places with minimal border control, like the U.S. before the world wars. Were there places a hundred years ago and earlier where boarders were guarded like they are today? Did they want to keep the serfs in place? Did rulers care about peasants moving back and forth? Weren't boarders usually less populated areas, with nobody keeping track of crossings? Or are modern borders, imperfect as they are, only possible at this time due to modern governments and the military resources they can dedicate to such things? My guess is that modern borders and modern passports came about around the same time. There's no sense in requiring documentation for every person crossing a border unless a country has the wherewithal to enforce that rule.