You've forgotten that the game starts with the Fallen hunting you. The Fallen have attacked the city in the past. They've occupied the zones around the walls. The Fallen are openly hostile to Guardians and teh City and have been since long before any events of the game. The Cabal may not actually be an enemy, but they occupied a former human world and set up a blockade to keep humans from getting in. That's why the Dead Orbit fleet can't go to Mars. The Vex haven't directly attacked the City, but they've taken and transformed several human worlds, including Venus, Mars and Mercury. On Venus the Vex apparently killed the human population. It's possible (and even suggested) that the reason there is only one human city left is because of Vex time manipulation. The Hive invaded and took the Moon from Humanity, long before the events of the game. During the events of the game our Ghost says that we "awoke the Hive", but this isn't true at all. The hIve had already begun a covert invasion of Earth in search of Rasputin.