rock and a hard place
We're unlikely to ever know if there was any heavy-handed blackmail applied to Powell, but I think his situation was enough to force him to go along. On top of his duty to honor his pledge of loyalty to Bush, I'm sure in the back of his mind were his presidential aspirations. No way he could've forgotten the talk after the Gulf War of drafting him as a Presidential candidate. He couldn't get himself fired from the Bush Administration, and had to wait for the customary second-term turnover to make as graceful an escape as possible. Too late to salvage his reputation, but to his credit he did resign at the earliest opportunity.
I think that's what happened. I've no doubt that Powell regrets hitching his star to Bush.