In ancient times they were the traveling caravans. Merchants who went from town to town, country to country. Access to opium, wine, even pornography. They could destabilize a country by flooding it with these. They were often a source of news from distant places and could spread rumors of far-off lands plotting invasion. They could, of course, loan money, and sell weapons too. They were allied with international priesthoods that'd institute religions saying bury the dead with lots of gold, then rob the graves and smuggle the treasure out of the country and melt it down. By the way, citizens, keep all your gold in the temple for safekeeping and use our fiat currency. Then they'd steal that gold one day and move it out of the country, it all having being planned in advance yet nobody being the wiser. Yes, this system goes far, far back.