Gandalf Could've Killed Sauron Long Before LotR; Does It Mean It Was All His Fault?
Or, at the very least, he could have prevented Sauron from returning to power as smoothly, as the latter did – after all, while the One Ring existed, Sauron could not truly perish, despite repeated destruction of his physical bodies.
Gandalf, like his fellow wizards, was dispatched to Middle-earth with the sole purpose of hunting Sauron. And to his credit, he did a better job than others.
While Middle-earth had grown complacent, elves mostly believing that Sauron would never return, and other wizards focusing on their favorite personal activities, Gandalf remained ever vigilant.
He investigated every lead and made connections all around the kingdoms. But at one point he committed a crucial blunder.
Many centuries after losing the One Ring, Sauron began re-establishing himself in Middle-earth, Weakened as he was, he did not announce himself openly, and operated by subterfuge, sending his Ringwraith servants to raise kingdoms and armies in his stead, while manipulating orcs and men of the East into attacking his enemies, slowly grinding them down over millennia.
He had hunkered down at the isolated fortress of Dol Guldur and kept a relatively low profile, becoming known as "The Necromancer".
It wasn't until 150 years before The Lord of the Rings when Gandalf finally managed to infiltrate Dol Guldur, where his suspicions were confirmed – Sauron was back.
Gandalf rushed to Rivendell and reported his finding to the White Council, a group of wizards and ancient elves who watched over Middle-earth.
Yet the council's leader was Saruman, who at that point already must have contemplated treachery, which he later committed.
And Saruman advised against taking action. He persuaded everyone that Sauron could never regain his full strength, and so immediate war was not necessary.
In their complacency, the rest of the White Council agreed with him. And in a rare moment of foolishness, Gandalf allowed himself to be persuaded as well.
They left Sauron alone for 100 years, giving him plenty of time to finalize his preparations.
So when, at the time of The Hobbit, they've finally managed to chase Sauron out of Dol Guldur, he was ready to simply move into his old stronghold of Mordor, surround himself by armies and thralls, and proclaim his bid for domination of Middle-earth for all to hear.
Sauron's servants even re-occupied Dol Guldur a few years later.
So, while destroying Sauron might have been beyond Gandalf's ability, he certainly missed a chance to deal Sauron's plans a crippling blow.