Final answer:
Iago's manipulation is the spark that leads to the downfall of Desdemona and Othello in Shakespeare's play, causing Othello's jealousy to erupt and setting forth a tragic chain of events.
Step-by-step explanation:
The start of Desdemona and Othello's downfall in Shakespeare's play Othello is rooted in a combination of factors, including Othello's jealousy, Desdemona's innocence, Iago's manipulation, and external circumstances such as Cassio's promotion. However, among these elements, it is Iago's manipulation that ultimately sets the tragic events in motion. Iago's cunning exploits Othello's vulnerability and triggers his jealousy, leading to the catastrophic collapse of their marriage and ultimately their demise.
Iago's deceitful actions are engineered to create mistrust and spur Othello into a jealous rage, capitalizing on both Othello's insecurities and Desdemona's naivety. The play highlights the dangers of unchecked emotions and the impact of manipulation, with Iago's orchestration of Othello's suspicions serving as the fulcrum upon which the tragedy tilts into irreversible tragedy.