system
Given the following post, respond as if you're commenting on Reddit:
human
[Dune] House Atreides is relocated to Arrakis, knows it's a trap, and yet

... their ships are on the ground in the same place, their troops all seem to sleep at the same time, it takes them minutes and minutes to mobilize, and there seem to be a few thousand troops to defend a duke at the most coveted location in the known universe. 

Granted, the loss of the shields due to treason was unforeseen, but I can't imagine, knowing of the impending threat, putting all of my ships conveniently within the same few square miles, not having military on rotations, and not having adequate defenses at all times. Why not have patrols near where the Spacing Guild might drop off accursed Harkonnens? Why not have the Duke at a more secret location? Why only have only what looks like maybe a brigade or two instead of having a full field army or navy? 

With the backing of the Emperor, perhaps it was a lost cause no matter what, but the loss was so absolutely devastating that it seems like there was more than just the negligence of trusting a doctor who's love for his wife superseded his loyalty.
gpt
There were multiple unforeseen factors that caught the Duke completely off guard. Doctor Yueh betraying them was unprecedented and robbed them of 90% of their defenses outright, but the major x-factor was the Sardaukar getting involved so quickly in tandem with the Harkonen armies.

The Duke assumed a trap but also assumed he'd have more time to prepare and was training troops on par with the Sardaukar. The Emperor knew this and wanted him out of the great game *immediately* and the Harkonens were willing to bankrupt themselves to get the chance to win their rivalry outright, both of which are extreme moves that there was no reasonable cause for thinking was going to happen.