system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR'S ROTK Film] If the army of the dead hadn't existed ( they sure did come in handy ) would the Fellowship still have won the war or would they have lost the battle and plunged Middle Earth in to darkness with or without the Ring?
gpt
If there had been no second wave of reinforcements to aid Minas Tirith, then it would be likely that Minas Tirith would have fallen on that day. Remember, Aragorn's party not only provided relief to the defenders, but also prevented the Corsairs from joining the fight. 

Setting aside the outcome of the battle of the Pelennor fields, Sauron's total military strength (of which only a fraction was committed to Pelennor) was actually so great that there was no chance of victory in war for the free peoples. Even Elrond and Galadriel knew it was hopeless to try and resist forever.

Coming back to Minas Tirith, this is why it was crucial for Aragorn to bait Sauron's reserves into leaving. The free people's only hope was for Frodo to complete the mission. It would be impossible to bait Sauron's reserves if both Minas Tirith is besieged and Sauron's main army isn't destroyed. So something the like Rohirrim abandoning the fight to try and draw out Sauron's reserves at the Black Gate won't work. Sauron's main army would simply give chase, and Mordor would remain filled with Orcs.