Re: Gregor & Tarly trapping the northmen, I’m getting maybe Tarly isn’t quite a military genius, but that seems like obvious tactics. Which do you think is more likely the cause? Gregor charging into the retreat path Tarly would have left out of bloodlust (or Tywin’s vindictive orders), or Tarly being affected by Robert’s successful retreat, maybe taking the lesson that you can’t count on an ally to finish a defeated foe, and not want Glover & Tallheart to rally elsewhere? Or some other factor?

Well, it’s not such obvious tactics, because outside of some of the best premodern armies ever (the Byzantines, the Mongols), people usually went for the complete encirclement. But there are other things that could have happened:

  • Gregor charging in recklessly is quite in character – his wing of the battle takes the heaviest casualties in the Battle of the Green Fork, he loses half his men at the Battle of the Fords, etc. 
  • It could be that Robett Glover is good enough of a commander that he reacted well enough to a bad situation to make the Lannisters pay dearly for their victory.
  • Or something else. 

How did the Lannister/Tyrell forces at the Battle of Duskendale manage to take heavy casualties given the massive advantages they were given?

Great question!

Sweeping victories in warfare tend to be rather rare, and even more rare without cost. Cannae is deservedly famous in military history as a complete victory, but even that total rout still cost Hannibal more than 10% of his army. 

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For this very reason, the Byzantine manual of war (hat tip to @warsofasoiaf) advised against completely surrounding one’s enemy, because men who are completely surrounded will often fight like cornered rats and inflict surprisingly heavy casualties. Instead, the Strategikon advised that one should always leave a path of escape for the enemy, because then rather than standing their ground, the enemy was more likely to make a run for it, which would allow one to inflict heavy casualites on a fleeing enemy at a lower cost to oneself. 

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Indeed, as military historians have pointed out, it was often the retreat from a battle that was the most deadly part of any premodern conflict, because a fleeing enemy couldn’t defend themselves against attacks coming from behind them, and often threw away heavy shields and armor in the process of flight. 

My guess is that Randyll Tarly and Gregor Clegane didn’t think about this.