[@Abefroeman]


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I think it was more a matter of them being caught off guard. Clearly, the Tyrell armies have some bite to them - if only by sheer size, but from the looks of things the Lannisters caught them [i]completely[/i] off guard. Highgarden probably had a minimal garrison at best, and I wouldn't be surprised if the Tyrell forces were off somewhere completely different, mustering to repel a more... conventional assault - rather than something that deep within their territory. 

Of course, Tywin's training certainly would have helped a great deal - Mace Tyrell was kind of... a crap military commander, to put it bluntly, and if he was in charge of the overall training, I wouldn't be surprised in the slightest if that meant the Reach armies were sorely outmatched by those of the Westerlands. However, we don't know for sure how extensive Mace Tyrell's incompetence extended, sooo...
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