Deliberately, their premise was flawed. They hinged their success on killing the Night King, something they could only do if he exposed himself, if they could kill him at all. Everyone else was anticipated to just fall back to a series of positions, expecting to be overrun. This plan fell apart right away, except for the plan to fall back expecting to die – that part went just as they all thought it would.
1. They deployed their maneuver forces first.
Not only did they send the Dothraki group against the undead, the Dothraki were sent charging in determined against an enemy they couldn’t even see. The Dothraki have zero experience fighting in the dark, in the cold, or against an army that isn’t already frightened of them by the time they arrive.
There was no reason to send them into the fighting first or to depend on them to do much damage to an devastating undead wave. Reliance on maneuvering troops in an excessively surrounded stronghold is what ended the French Army in Indochina, and it almost ended the army of the living.
2. They made little use of air superiority.
Everyone talks about these dragons as if they’re going to level the playing field or give Daenerys Targaryen the perpetual upper hand. So what were Daenerys and Jon Snow waiting for? Dany was the least disciplined person on their side anyway, so once the plan went out the window, the dragons should have been playing tic-tac-toe all over the undead horde.
The enemy dragon didn’t show up until halfway through the battle and was using undead dragonfire like it was the key to beating the living because it was.
3. They had no eyes on the battlefield.
Every time the dragons lit up part of the enemy, it not only took enemy soldiers off the battlefield but it gave them living targets for their artillery and archers. A vast chunk of Winterfell’s defenders were barely used because they couldn’t see the incoming enemy. The Dothraki rode straight into the swarm, swiftly overrun by a force they couldn’t fight because they couldn’t see them.
The only time the living army had any type of opportunity or was able to use their natural abilities to their benefit was when they could see the enemy to shoot at them. Ask Theon Greyjoy and the crew from the Iron Islands as they stood around defending the group project’s least productive partner. They made every arrow count.
If Arya Stark hadn’t actually killed the Night King, then Melisandre would have to be Winterfell’s MVP – she really gave the defenders light to see.
4. They failed to plan for the enemy’s reserves.
All the Night King had to do was raise his arms by 90 degrees to bring in an completely new wave of fresh troops to finish off whoever was left standing among the living. No fewer than 10 of the Winterfell defenders knew this, but failed to relay that message. Would it be so tough to take a swing at a corpse with your dragonglass just to make sure you don’t have to fight your friend later on?
Still, everyone was amazed and overwhelmed when the Night King raised the dead. Especially those who decided to hide out in a crypt.
5. The living still somehow managed to underestimate their enemy.
As Jon Snow ran up behind the Night King, the enemy leader stopped, turned, and raised another army of the dead. Jon Snow seemed very surprised by this. Why wasn’t the Night King giving him the one-on-one duel of honor Jon Snow knows he deserved? Because the Night King doesn’t care about things like that.
All he does is win. He has no problems with winning a lopsided fight, even if he never has to fight it himself. Jon and Daenerys thought they could just swoop down and kill the night king with dragonfire, despite there being a huge lack of proof that he could be killed at all, let alone with fire. Then they assumed he would just disclose himself and let himself to get splattered with fire.
In their plan, every minute they didn’t know where the Night King was hiding or flying, there were hundreds of troops fighting for their lives and souls. Every minute their dragons weren’t spewing fire on anything else, the Night King was heavily recruiting for the White Walker Army Reserve.
Thank the old gods and the new for Arya Stark. Somewhere, CIA agents from the 1960s are nodding their heads in approval.