Sessions loved the Department of Justice as an institution and trusted its vaunted "career professionals," of which he was once one as a young lawyer. Thus, as AG, Sessions was naturally inclined to let himself be led by the nose, taking resumes and recommendations at face value and making key personnel choices as if he a new dean selecting bright and congenial staff for a prestigious law school that had a few openings.
Much more was needed though. Due to the intense and unprincipled opposition to Trump, his choice as AG would be called upon to seize, occupy, and govern choice bureaucratic territory held by his enemies. Doing so would require a constant battle. Sessions though was temperamentally incapable of such a role, and especially so after the brutal treatment dished out during the fight over his confirmation.
Instead of a loyal and resolute AG who was a reformer with a street fighter's realism and toughness, in Sessions, Trump got a clueless fuddy-duddy who entered office suffering a form of PTSD after the rough handling he got from Trump's enemies during Senate confirmation. Instead of a pit bull fighting to advance the Trump administration's interests and policies, in AG Jeff Sessions, Trump got an old, whipped dog desperate for affection, peace, and comfort.
Under easier and better circumstances, Sessions could have been a fine AG. Or, if Trump had sent Sessions over to the DOJ with a phalanx of tough political combatants authorized and determined to clean house, Sessions might have made it through a term as reasonably good AG. Yet that was not to be, and Sessions failed as AG due to his inadequacies.
Under Sessions, Rosenstein and other villains in the DOJ smiled to his face, snickered behind his back, and broke the law as they intrigued to use the FBI and the DOJ to bring down a President. Trump had good reason to be disappointed and furious with Sessions, and, I suspect, sad for him as well.
Congratulations, your post is probably the best description of what may have actually happened that I have seen, in years of looking for an answer.
It mostly blames Sessions, it lightly blames Trump, and describes a perfect storm.
However, the destruction to the country, and the pain it caused was immense. At some point, one of them should have recognized what was going on, and done something about it.
Sessions, being in the subservant role, should have fallen on his sword if necessary, as soon as the Strozk texts came to light. Instead, he dug in his heels, and argued with Trump publicly. He clung to his appointment that he had already tarnished irreparably.
Sessions failed, ultimately, and horribly. Trump survived, no thanks to Sessions, but the scars on his Presidency remain, and may never go away.