Final answer:
The downfall in Shakespeare's Othello begins with Iago's manipulation, which triggers the chain of events leading to the tragedy.
Step-by-step explanation:
The start of the downfall in Shakespeare's Othello is Iago's manipulation. Although there are multiple factors that contribute to the tragic events in the play, including Othello's jealousy, Desdemona's alleged betrayal, and Cassio's promotion, it is Iago's cunning manipulation of all characters that sets the tragic events into motion.
His actions are instrumental in instigating Othello's jealousy and misconceptions towards Desdemona, ultimately leading to the tragedy of the play.