Trōiānī vērō, cum Graecōs Argōs in patriam suam āvectōs esse arbitrārentur, tum dēmum post tot annōs portās aperuērunt atque exīre ausī sunt.
as is annotated, Argī is a city, but I can hardly understand the cum-clause:
The Trojans in fact, not only thought Greek Argi(?) had been driven away to their homeland, but also finally opened the gates and dared to go out after many years.